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Lat/Long projected/unprojected   Craig Eissler Jun 05, 2003
Re: Lat/Long projected/unprojected   Craig Feuerherdt Jun 05, 2003
Re: Lat/Long projected/unprojected   Melita Kennedy Jun 06, 2003
Re: Lat/Long projected/unprojected   Craig Eissler Jun 06, 2003
Re: Lat/Long projected/unprojected   Melita Kennedy Jun 06, 2003
Re: Lat/Long projected/unprojected   Craig Eissler Jun 06, 2003
Re: Lat/Long projected/unprojected   Melita Kennedy Jun 06, 2003
Re: Lat/Long projected/unprojected   Craig Eissler Jun 10, 2003
Re: Lat/Long projected/unprojected   Justin Johnson Jun 12, 2003
Re: Lat/Long projected/unprojected   Melita Kennedy Jun 12, 2003
Re: Lat/Long projected/unprojected   Justin Johnson Jun 12, 2003
Re: Lat/Long projected/unprojected   Melita Kennedy Jun 12, 2003
Re: Lat/Long projected/unprojected   Justin Johnson Jun 12, 2003
Re: Lat/Long projected/unprojected   William Huber Jun 13, 2003
Re: Lat/Long projected/unprojected   Justin Johnson Jun 13, 2003
Re: Lat/Long projected/unprojected   William Huber Jun 13, 2003
Re: Lat/Long projected/unprojected   Craig Eissler Jun 16, 2003
Re: Lat/Long projected/unprojected   Gabriel Montero Aug 08, 2003
Re: Lat/Long projected/unprojected   Melita Kennedy Aug 08, 2003
Re: Lat/Long projected/unprojected   David Lowery Aug 21, 2006
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Subject Lat/Long projected/unprojected 
Author Craig Eissler 
Date Jun 05, 2003 
Message ...this is more of a concept question, but pertaining to projections...

If converting coordinates from a 3D earth to 2D planar map requires a "projection", then why is a 2D map (in GIS on a "flat" screen) using Lat/Long (Geog Coord Syst) considered "unprojected"? ...We flattened the earth, correct?

In other words, "when" does the PROJECTION actually take place? ...Thanks, I'm trying to explain this to my students. 
   
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Subject Re: Lat/Long projected/unprojected 
Author Craig Feuerherdt 
Date Jun 05, 2003 
Message Craig,
In order to get a better understanding of projections etc I would be inclined to look around on the web or find an appropriate text book - something like 'Elements of Cartography'.
Craig 
  Craig F
GIS Analyst 
   
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Subject Re: Lat/Long projected/unprojected 
Author Melita Kennedy 
Date Jun 06, 2003 
Message You're correct, data in a geographic coordinate system is being
projected when we display on a computer screen (or print it).
Basically, it's the Plate Carree projection. Normally data is Plate
Carree is also multiplied by the spheroid's semimajor axis to
convert the values to meters.

No matter how it's displayed (temporarily), it's still being stored
in a geographic coordinate system.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Lat/Long projected/unprojected 
Author Craig Eissler 
Date Jun 06, 2003 
Message Thanks, Melita.
So...

Lat/Long coordinates (in Plate Carree, or perhaps some other projection) is indeed "projected" if we have flattened the spatial data to represent it accordingly on a computer screen (in GIS) or paper map.

If so, what explains calling this Lat/Long data "unprojected", as I believe ArcView does?

Along those lines, don't we make a distinction between a 'Geographic Coordinate System' and 'Projected Coordinate Systems'? ...If so, what now constitutes the former?

Thanks again, Craig
 
   
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Subject Re: Lat/Long projected/unprojected 
Author Melita Kennedy 
Date Jun 06, 2003 
Message Because the data is still stored in a geographic (unprojected) coordinate
system.

We do now make a distinction between a 'geographic coordinate system'
and a 'projected coordinate system'. Data in a geographic coordinate
system have coordinates that are measured using an angular unit of
measure, usually degrees.

To display it, we treat the angular unit of measure as if it's really a linear
unit of measure. The closest projection to this is Plate Carree, but if you
really projected your data using the Plate Carree map projection, the XY
coordinates would be scaled much larger because the lat/lon values are
multiplied by the semimajor axis of the spheroid (of the geographic coordinate
system).

Many of the analysis tools are designed to use cartesian (2D) mathematics.
That's why buffers, some distance calculations, comparisons of areas
should occur with data in a projected coordinate system. A map projection
does the conversion with known distortions that can be calculated (we
plan to add this in the future). 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Lat/Long projected/unprojected 
Author Craig Eissler 
Date Jun 06, 2003 
Message ...OK, is it fair to say then that there is some general contradiction in the term "projection" as it is used in GIS..? Or am I getting this all wrong?

In other words, are we saying that Lat/Longs are displayed in ArcView through a 'projection' using Plate Carree, but is NOT considered a 'projected coordinate system' because the coordinate data is still Lat/Long, rather than X/Y?

If so, are we saying then that data can be projected without being in a projected coordinate system? ...huh?

Please help clarify this concept, so that I can clarify it to my students.

Thanks, Craig
 
   
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Subject Re: Lat/Long projected/unprojected 
Author Melita Kennedy 
Date Jun 06, 2003 
Message You are likely correct that the terminology is confusing and probably
sometimes used incorrectly.

Data in a geographic coordinate system is displayed by treating the
angular unit of measure as if it's really a linear unit of measure. The
closest projection to this is Plate Carree.

All data is transformed to display on a computer screen--into pixels.
Perhaps I should have said transformed or converted.

No, the display of geographic data, or setting the coordinate system
of the data frame to a geographic coordinate system, is not considered
a projected coordinate system. If you save the data out to a new
feature class or shapefile, the data will be stored in lat-lon, a
geographic coordinate system.

Let's ignore the fact that you can transform data on-the-fly to a new
coordinate system (often called 'project on-the-fly' even if the output
coordinate system is a geographic one).

When we display any spatial data in the data frame, we do a transformation
to the Map space (where the upper left corner is 0,0) and then to the
screen's pixel-based coordinate system. Luckily, most people (including
me!) don't need to deal with this. ArcMap deals with all this and reports
coordinates based on the actual values of the data whether it's in a
geographic or projected coordinate system. All the data is treated the same,
as if it's in a 2D cartesian system. In many ways, this is incorrect for
data in a geographic coordinate system. We should show it as a globe but
that's harder, so the convention is to treat it as if it's 2D. Because a
projected coordinate system is a 2D cartesian system, the terms are
often used interchangeably.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Lat/Long projected/unprojected 
Author Craig Eissler 
Date Jun 10, 2003 
Message Thanks, Melita.

I think this is information I can now use to explain to my audience "how it works".

...By the way, I hope you can also complete our dialog on the other Discussion topic about Georeferencing/Orthophotos -- thanks.

-Craig 
   
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Subject Re: Lat/Long projected/unprojected 
Author Justin Johnson 
Date Jun 12, 2003 
Message My main area of confusion has to do with the way coordinate systems and projections seem to be mixed together.

As I understand it, a geographic coordinate system stores coordinates using lat/long. The values of the coordinates for a particular location will change according to the datum, but the projection has no effect on the values and is only related to the presentation of the data. It's sort of like changing the font. The projection has no effect on the spelling of the words, only the way they look on paper.

A projected coordinate system measures locations on a projected surface, such as a Lambert Conformal Conic or a Transverse Mercator in the case of State Plane or UTM. Here, the values do change based on the parameters of the projected surface upon which they're measured. My current location in UTM zone 12 will be completely different if I try to measure it using the parameters of zone 13.

The confusion seems to be with the way coordinate systems, datums, and projections are all mixed together under two categories in ArcGIS: Projected Coordinate Systems and Geographic Coordinate Systems.

Under the Projected Coordinate Systems category for North America, there are options such as "North America Albers Equal Area Conic" and "North America Lambert Conformal Conic" I would argue that these are purely projections and should not be lumped in with the coordinate systems. A point's lat/long values in the coordinate system stay the same no matter which of these options is selected. The appearance and other properties of the map change when the projection changes, but there is no change in any of the values in the coordinate system when the projection changes.

It seems like there should be three lists to select from: the coordinate system (lat/long, UTM, State Plane, etc.) which defines the way the location values are recorded; the datum, which describes the surface upon which they were measured; and then the projection of the map, which determines how the map is displayed on screen or paper.

Just a thought.

 
  Justin Johnson
SLC, UT 
   
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Subject Re: Lat/Long projected/unprojected 
Author Melita Kennedy 
Date Jun 12, 2003 
Message I think a lot of the confusion comes from the fact that ArcInfo
workstation used (uses) the term 'projection' both when it's
referring to a coordinate system (geographic and projected!),
and the map projection itself.

We've tried to move away from that in ArcGIS, but it's
difficult to change usages consistently and completely.

To me, a map projection refers to a set of equations that can
convert between longitude and latitude values and xy values.
Mercator, Transverse Mercator, Lambert Conformal Conic, etc
are all map projections.

When we use a map projection as part of a projected coordinate
system, we have to set any parameters needed by the map projection
(scale factor, central meridian, whatever), plus include a geographic
coordinate system.

The prj files in the Coordinate Systems directory are all
coordinate systems, not projections. The example you gave,
North America Lambert Conformal Conic, is a projected
coordinate system using the Lambert Conformal Conic map
projection which has parameters and a geog coordsys set
for North America. I suppose I could have named it
"North America Lambert Conformal Conic Coordinate System"
but that's a bit wordy.

Melita
 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Lat/Long projected/unprojected 
Author Justin Johnson 
Date Jun 12, 2003 
Message There is a lot of confusion and misuse of the term "projection." I hear people talk about setting the projection on their GPS unit when they really mean setting the coordinate system or datum.

In keeping with that confusion, I assumed that the difference between a Geographic Coordinate System and a Projected Coordinate System was the surface on which the measurements were made. I thought that a Geographic Coordinate System measured locations on a representation of the earth using the angles above or below the equator and east or west of a particular prime meridian and that a Projected Coordinate System used a flat projected surface and cartesian coordinates with an origin on that surface to define a location. whew.

I see the definition of North America Lambert Conformal Conic as a particular instance of a LCC that has two standard parallels selected to minimize the angular distortion on a map of North America. But, this configuration doesn't have any effect on the coordinates. They stay the same unless a parameter of the coordinate system changes, i.e. the location of the Prime Meridian or the shape of the spheroid. In the case of a projected coordinate system, if I changed the the standard parallels of the Utah Central state plane zone (which uses Lambert Conformal Conic), all of the point locations in that zone would change because that coordinate system is based on that projected surface.

But, if by "Projected Coordinate System" you mean a specific coordinate system projected in a specific way, then I can see how North America Lambert Conformal Conic could be a projected coordinate system and you can disregard the previous paragraphs. Although, if that's the case, you may want to consider calling it a "Projection of a Coordinate System".

I'll stop now before this is nit-picked to death.


 
  Justin Johnson
SLC, UT 
   
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Subject Re: Lat/Long projected/unprojected 
Author Melita Kennedy 
Date Jun 12, 2003 
Message We agree on the difference between geographic coordsys (geogcs) and
projected coordinate systems (projcs)--the first uses angles to locate
points while the second locates points in a flat, Cartesian plane.

I really want to understand why you believe "NAmer LCC" (which uses
the NAD83 geographic coordinate system) is different than "NAD83 State
Plane Utah Central". It will help me explain coordinate systems in the
future.

Both are projected coordinate systems. In fact, "Utah Central State Plane"
isn't enough information to determine which projected coordinate system
it is. Is it based on NAD27, NAD83, or NAD83 HARN? If NAD83, is the linear
unit of measure the meter or foot?

If I have data that is permanent and based on the North America Lambert
Conformal Conic, the coordinate values will change if I change any of the
projection parameters or the geographic coordinate system (to change the
prime meridian), the same way data based on a State Plane zone would
change.

I don't know if the following will help, or confuse the issue.

A geographic coordinate system is a reference frame for the world as a
globe (spheroid). Particularly for a large area, it's closer to reality.

If you're working in a small area, it's easy to treat the world as a flat
plane, and if you don't need to work with other data, you can use
a local coordinate system. If you want to be able to reliably relate
the data to other data, you usually need to use a well-defined
projected coordinate system.

Because the math is well-defined, we can take data in a projected
coordinate system and unproject it to its inherent geographic coordinate
system, or to a completely different projected coordinate system.

A projected coordinate system has to include a geographic coordinate
system. Most of the math for a map projection needs the spheroid
values, plus the prime meridian and angular unit of measure define
how the projection parameters are defined (central meridian is
relative to the prime meridian, if angular unit is degres, central meridian
is in degrees, etc).

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Lat/Long projected/unprojected 
Author Justin Johnson 
Date Jun 12, 2003 
Message I can see a little better what you mean by Projected Coordinate System now. Any projected coordinate system has to include a geographic coordinate system in order to get standard parallels or a central meridian, or any other basic element of a projection.

The main difference that I see between a geographic and a projected coordinate system is that geographic coordinates are not dependent upon a projected surface. They are independent of a projection. But, a projected coordinate system relies on a well-defined projected surface upon which to base the cartesian coordinates.

If I have a list of geographic coordinates for points along the California coast, I could feed those coordinates into the formulas for a Mercator projection, a Gnomonic projection, Lambert Conformal Conic, etc. without having any effect on the values of the coordinates themselves. The resulting maps would have different properties and appearances, but the coordinates would not change.

If I set out mapping the coast of California in UTM zone 10, I'm going to end up with different values for each location than I would have if I mapped the coast using UTM zone 11. Each of those zones is based on a projected surface set up over a different central meridian. So UTM, as I understand it, is a projected coordinate system because the coordinate values are dependent on a specific defined projection.

The Utah Central State Plane zones vary because different datums are used when establishing the projected surface. The location on the ground of the central meridian, standard parallels, and point of origin will vary between NAD27 and NAD83. Once the surface and point of origin on that surface are established, you can measure your northing and easting using any distance units.

I wouldn't consider North American LCC to be a coordinate system. I would consider it to be a projection of a geographic coordinate system. The two standard parallels were probably selected to minimize distortion over North America, but that's just a property of the projection. You could do the same thing for Europe, Australia or Texas. In order for it to be a coordinate system, I would have to be able to define a location somewhere based on it. I can't say that I'm currently located at X and Y based on North America Lambert Conformal Conic. I can say that I'm located at X and Y based on UTM zone 12 or Utah Central State Plane NAD83 or that my latitude is Y and my longitude is X based on NAD83.

I guess I'm breaking it up more than it may need to be. I'm sort of thinking that the only Geographic Coordinate System is lat/long, since it's based on a representation of the earth. Changing the datum or the central meridian will change a point's values, but projecting those points will do nothing to the values. Changing the underlying surface of a projected coordinate system changes the values of locations measured in that system.

That was the main distinction I always made. A projection is a way to represent a Geographic Coordinate System. A projected a surface is the foundation of a Projected Coordinate System. And both are dependent on a datum.

 
  Justin Johnson
SLC, UT 
   
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Subject Re: Lat/Long projected/unprojected 
Author William Huber 
Date Jun 13, 2003 
Message It might help to have a more general understanding of coordinate systems and projections, because that will put the earth-centric situation into a larger perspective. I would like to share some conventional mathematical definitions, because they offer the potential to clarify things. The following discussion applies to arbitrary surfaces as well as higher-dimensional spaces, but I will use language specific to the problem of mapping the earth.

There are three mathematical elements to a system of geographic reference: a projection and *two* coordinate systems. After defining and describing these, I will provide a few simple examples.

(i) There is a mathematical transformation, the “projection,” between an idealized surface representing the earth (the ellipsoid) and the Cartesian plane. This transformation does not require either surface to have coordinates. We are all aware that some transformations can be defined geometrically, for instance (stereographic, equal-area cylindrical, and conic projections come to mind); these provide examples where coordinates are not needed. In most situations, though, a simple geometric definition is not possible (or has not been discovered).

(ii) There are coordinates for the earth's surface. There are two fundamentally different methods used. One is _intrinsic_: a unique point on the earth’s surface is associated to each pair of real numbers that are “valid” coordinates. Key things to note about this definition include

(a) The *inverses* of the coordinate functions are a pair of real-valued functions defined on the surface. They have to satisfy certain topological and differential properties that I won't go into here; suffice it to say that they must be "well behaved."

(b) The coordinate functions do not have to cover the entire surface. This implies any coordinate system might be limited in coverage. The most common example is lat-lon. Longitude and latitude are the *inverses* of the coordinates, because they are functions assigning coordinates to points on the ellipsoid, rather than functions assigning points to coordinates. Longitude is *undefined* at the poles. This causes problems when people forget this and try to use lat-lon for calculations near the poles. Another common example is a “local” or “site-specific” coordinate system set up in some small region; there never is any intention to use such systems to reference points over the entire earth.

The usual intrinsic coordinates for an ellipsoid are based on lat-lon, but many others are also in common use. For these functions to be useful, you need a _datum_. This is the information needed to match a physical location to any pair of coordinates. It usually includes an explicit model of the ellipsoid, an origin (a distinguished point on the ellipsoid), and a distinguished direction at the origin. The need for a datum is clear mathematically: the coordinates give us a point on an abstract ellipsoid, but how exactly does that ellipsoid correspond to the earth itself? That is a matter for a surveyor to determine.

The other kind of coordinates are _extrinsic_: they are coordinates for a region of space surrounding the earth. Unlike property (b) of intrinsic coordinate, extrinsic coordinates usually cover every point of the earth. Again a datum--albeit of a different sort--is needed to associate coordinates with physical points. The datum consists of an origin, often taken to be the center of the earth, and three vectors at that origin defining three distinct directions (and distances), often taken to be rotating with the earth. What is interesting about these is that most coordinate combinations do not correspond to points on the earth’s surface, but rather to points above or below it. Given specific coordinate values (x,y,z), it is not usually easy to tell whether they correspond to a point on the earth’s surface or not: you have to do a calculation.

(iii) There are coordinates in the Cartesian plane. Many types of system exist: rectangular, polar, confocal elliptical, etc. In addition to the type of coordinates, these come with a datum of their own: the origin, orientation, and scale. Think of the plane as a blank sheet of paper; the datum is established when you draw two coordinate axes and mark off scales on them (for rectangular coordinates) or when you draw an origin and a direction at that origin denoting the zero angle (for polar coordinates) and mark off a scale along that direction.

There are interactions among these two coordinate systems and the projection. For example, rescaling the coordinates on the earth and rescaling the coordinates on the plane can accomplish the same thing, mathematically. This can cause some confusion.

As another example, the "false easting" or "false northing" sometimes applied to location data are changes of datum *in the plane*, not on the ellipsoid. This shows as clearly as anything that there really are two coordinate systems involved in any useful map of the earth.

Coordinates (ii) and (iii) are needed for several reasons. In addition to providing names for points on the earth and the map, respectively, they also provide a mechanism to write down projections in terms of formulas. Obviously we cannot write a formula for a projection until we have numbers for referring both to points on the earth's surface and points on the map.

Here is an example. The ESRI 'geographic' coordinate system (i) is a Plate Carree projection, (ii) uses lat-lon in decimal degrees [the coordinates] relative to the Greenwich prime meridian on a spherical earth model [the datum], and (iii) uses rectangular coordinates in the Cartesian plane. If we let (lambda, phi) be the lon-lat coordinates on the earth and (x, y) be the planar coordinates, then the projection has an extremely simple definition: y = phi and x = lambda. By equating the coordinates in this way we can be misled into thinking there is 'no projection,' but that evidently is not true.

As another example, let's track the process of locating a physical point on the earth based on map coordinates. Beginning with coordinates (x, y), one uses the map's datum and coordinate system to find a point on the map whose coordinates are (x, y). The projection formula (or rather its inverse) associates that point with a unique set of earth coordinates. The earth's datum and coordinate system tell us how to use those coordinates to go to a definite point on the earth's surface.

The datums associate the abstract mathematical entities--an ellipsoid and a plane--with physical points on the earth and the map, respectively. Everything else--the two coordinate systems and the projection--is purely mathematical.

I think it is helpful to bear in mind that every system of geographic reference necessarily involves a projection, *two* coordinate systems, and *two* datums. All these ingredients are always present, whether they are made explicit or not, and regardless of what terminology your GIS software or data vendor decides to use at the moment.
 
  --Bill Huber
Quantitative Decisions (http://www.quantdec.com )
More GIS Q&A at http://gis.stackexchange.com/q/3083/664 
   
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Subject Re: Lat/Long projected/unprojected 
Author Justin Johnson 
Date Jun 13, 2003 
Message
I can see how what I have been thinking of as a Projected Coordinate System meets your requirements of 2 datums, 2 coordinate systems, and 1 projection. That is, if the (X,Y) grid drawn on a sheet of paper and the mathematical plane of that sheet are considered to be a coordinate system and a datum.

But here's where I'm still thinking that a Geographic Coordinate System is more basic than that. I think that (and I'm no expert on this particular field) a Geographic Coordinate System requires one datum, one coordinate system, and no projection.

If I were to describe my location with a longitude and a latitude and tell you that it was measured on the WGS84 datum, my location in that coordinate space would be exactly defined. To display it on a map would require another datum, coordinate system, and projection, but that's irrelevant to my location. It is defined without the need of a projection. That's not the case with UTM or State Plane. They require a geographic coordinate system to define key locations on the earth, such as a central meridian or standard parallel(s), a datum to base that system on, and a projection to create the surface upon which the second datum and coordinate system are to be established and used to describe locations. The projection is a critical component of the coordinate system.

The Petrotechnical Open Standards Consortium uses definitions for Geographic/Projected coordinate systems that are similar to what I have been thinking.

Geographic: A curvilinear coordinate system based upon a geodetic datum. This coordinate system is defined by the axes of latitude, longitude and height. The latitude of a point is specified as an angle between the equatorial plane and a suitably chosen line thorough the point. The longitude of a point is specified as an angle between the local meridian of the point and the chosen reference meridian, measured in the equatorial plane. The height is the distance between a point and a vertical datum, such as the ellipsoidal surface associated with the geodetic datum.

Projected: A coordinate system that is a systematic representation of all or part of the surface of the earth on a plane based upon a projection coordinate transformation from a geographic coordinate system.

But those definitions could be less exact than what is ultimately required. It has just always been my understanding that a Geographic Coordinate System has no projection associated with it, only a datum and a coordinate system. This has been an interesting discussion. I hadn't thought about this in quite so much detail. Are we still on the original topic of this thread?

 
  Justin Johnson
SLC, UT 
   
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Subject Re: Lat/Long projected/unprojected 
Author William Huber 
Date Jun 13, 2003 
Message There's no problem in defining a "geographic coordinate system" as you do (no associated projection) and it makes sense.

The original question in this thread was,

"If converting coordinates from a 3D earth to 2D planar map requires a "projection", then why is a 2D map (in GIS on a "flat" screen) using Lat/Long (Geog Coord Syst) considered "unprojected"? ...We flattened the earth, correct?

"In other words, "when" does the PROJECTION actually take place?"

I believe my previous post completely answers that question. Parts of it might even be clear ... ;-)

As to the Consortium, its definitions, although intriguing, are not rigorous. Let's take a quick look.

"Geographic: A curvilinear coordinate system based upon a geodetic datum. This coordinate system is defined by the axes of latitude, longitude and height."

"Axes" is used in an unusual and undefined sense. An axis usually is a line or ray with a scale ruled upon it. As far as I know, defining lat, lon, and height requires much more than axes.

"The latitude of a point is specified as an angle between the equatorial plane and a suitably chosen line thorough the point."

"Suitably chosen" is too vague for a definition. (The correct definition of latitude is that it is the complement of the angle an outward-pointing normal vector makes to the normal of the equatorial plane.)

The restriction to lat-lon-height in this definition eliminates from consideration all geocentric coordinate systems as well as all other coordinate systems for the earth.

"Projected: A coordinate system that is a systematic representation of all or part of the surface of the earth on a plane based upon a projection coordinate transformation from a geographic coordinate system."

Here is the potential for confusion again. The initial "a coordinate system" implicitly means a coordinate system _for the earth's surface_. The fact is that all intrinsic coordinates for any surface *automatically* "represent all or part of the surface of the earth on a plane" by virtue of the fact that we can always interpret the two coordinate functions as Cartesian coordinates in the plane. Thus this entire phrase adds no meaning whatsoever to the definition. The word "projection" also adds nothing. Everything in this definition therefore is hidden in the meaning of "based upon". Here's a possible interpretation: the Consortium's "projected" coordinate system is the same as my intrinsic coordinates (with datum) for a surface.

So I agree: the Consortium's statements contain enough imprecision and vagueness to render them less than useful as definitions.
 
  --Bill Huber
Quantitative Decisions (http://www.quantdec.com )
More GIS Q&A at http://gis.stackexchange.com/q/3083/664 
   
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Subject Re: Lat/Long projected/unprojected 
Author Craig Eissler 
Date Jun 16, 2003 
Message Thanks for everyone's insight.

I recommend to ESRI that they no longer refer to the Geographic Coordinate System "unprojected", because it is projected in a GIS -- albeit for display purposes -- the data (Lat/Long) just hasn't been converted to a Projected Coord Syst (X/Y). ...please correct me if I'm still wrong.

...So I think this term (projection) is confusing for those studying concepts of mapping/geography and using GIS with ArcView simultaneously.

Thanks again, Craig
 
   
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Subject Re: Lat/Long projected/unprojected 
Author Gabriel Montero 
Date Aug 08, 2003 
Message I am confused about what coordinate system the "COVERAGE BOUNDARY" values are based on when you use the DESCRIBE command in Arc on a coverage. Apparently, this coordinate system is what projections are defined on, and even coverages with no projection defined have coverage boundary values. What's even more confusing is that Arc says "NO COORDINATE SYSTEM DEFINED" when there is no projection defined and then goes and gives you some values based on a coordinate system, but I think this confusion has already been pointed out. Anyway, is this "master coordinate system" the same as the Geographic Coordinate System? If so, why is it that the values based on this coordinate system are usually in the order of a million? Maybe the units are in ten thousandths of a degree? 
   
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Subject Re: Lat/Long projected/unprojected 
Author Melita Kennedy 
Date Aug 08, 2003 
Message The coverage boundary (extents) values are based on the actual
coordinate values of the data. Even though the coordinate
system is undefined, the data's xy values are still referenced to
something. It might be a geographic coordinate system or a
projected coordinate system.

Because the coordinate system is undefined, we don't know
whether the coordinate values are decimal degrees or meters
or feet or some other unit of measure.

When you're familiar with the coordinate systems that are used
in a particular area, you can often look at the data's extents
and decide whether the data is defined on a geographic coordinate
system, or UTM, or State Plane, or something else.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Lat/Long projected/unprojected 
Author David Lowery 
Date Aug 21, 2006 
Message I realise this is a rather late post and maybe off the question but this is really annoying me.

I'm happy with the definition of geographic as one datum no projection and projected as datum + mathmatical transformation/projection. What bothers me is in the metadata for projected data.

Let's say I have some data, It doesn't matter what it is (grid, vector, coverage), with a projection I know and have defined (could be UTM, lambert, whatever, units could even be standard snails if you wish). I accept that the bounding extent in the "projected co-ordinates" part will be in the datum and projection I specified (say Geodetic datum of Aust 1994 and UTM zone 55). WHAT datum is used to specify the dd co-ordinates in the geographic or "local" section of the maetadata.

I have not been able to find this data anywhere and if I want to report the extent as lat/long (Say you don't know what GDA94 UTM55 is and you can't get that information) I need that info.

Other than reprojecting my data to a geographic "projection" (say WGS84), what are my options. I would think if the system reports in dd it still should be stating the datum.

Just a thought.

David Lowery