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Longitude / Latitude (NAD83)   Andrew Smiarowski Dec 06, 2006
Re: Longitude / Latitude (NAD83)   Melita Kennedy Dec 06, 2006
Re: Longitude / Latitude (NAD83)   Andrew Smiarowski Dec 06, 2006
Re: Longitude / Latitude (NAD83)   Melita Kennedy Dec 06, 2006
Re: Longitude / Latitude (NAD83)   Kendis Scharenbroich Apr 30, 2007
Re: Longitude / Latitude (NAD83)   Melita Kennedy May 01, 2007
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Subject Longitude / Latitude (NAD83) 
Author Andrew Smiarowski 
Date Dec 06, 2006 
Message Which coordinate system is based on Longitude / Latitude (NAD83)only?
I am trying to use similar coordinate system to Longitude / Latitude (NAD83)that MapInfo users use.
 
   
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Subject Re: Longitude / Latitude (NAD83) 
Author Melita Kennedy 
Date Dec 06, 2006 
Message North American Datum 1983 (name: "GCS_North_American_1983") in the geographic coordinate systems, North America folder. Also called NAD 1983.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Longitude / Latitude (NAD83) 
Author Andrew Smiarowski 
Date Dec 06, 2006 
Message Well, in MapInfo if I use street network with NAD83 (latitude/longitude) when I want to measure the distance from one intersection to another one (going east/west, or north/south) it gives me accurate measurements as they are in reality.

In ArcMap 9.2 if I use GCS North American 1983, east/west measurements are not accurate.

For my location I use coordinate system NAD 1983 State Plane Illinois East Fips 1201 Feet, which gives me what I need but only for this area. If I do my work in a different state I have to change to a different coordinate system.

In MapInfo you did not have to do it if you were using NAD83 (latitude/longitude) and X and Y were in lat/long.

Is there a projected coordinate system where x/y are lat/lon, instead of feet?

Thank you,
 
   
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Subject Re: Longitude / Latitude (NAD83) 
Author Melita Kennedy 
Date Dec 06, 2006 
Message No, there isn't--that's the point of using a projected coordinate system.

MapInfo uses equidistant cylindrical for display even when the data is natively latitude-longitude. As you pan/zoom, the standard parallel of the projection is updated to help minimize distortion. In equidistant cylindrical, north-south distances and along the standard parallel (probably the center of the map) but not otherwise.

So the calculated distances are probably the equidistant cylindrical distances.

In ArcMap, if the data frame is using a geographic coordinate system and the distance units are linear (meters, feet), the geodesic distance is calculated. The geodesic distance is the shortest distance on the spheroid/ellipsoid. This is generally more accurate than distances calculated in when using a projected coordinate system.

If the data frame is using a projected coordinate system, by default, the calculated distance is the cartesian distance. You can override this to return the geodesic distance by holding down the shift key while measuring.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Longitude / Latitude (NAD83) 
Author Kendis Scharenbroich 
Date Apr 30, 2007 
Message I've been searching for hours and found this thread, maybe you can help me. I have data in lat/long, grs1980. If I'm using arcview 9.2 and performing buffers and measuring, is AV giving me accurate results? Do I need to project my data into a true projection system to get accurate measurements? Any insight is appreciated!

Thanks, K 
   
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Subject Re: Longitude / Latitude (NAD83) 
Author Melita Kennedy 
Date May 01, 2007 
Message In ArcMap, when the data frame is set to a geographic coordinate system (lat/lon), the measure tool will return the geodesic distance. That is, the shortest distance on the surface of the ellipsoid/spheroid.

If you measure the perimeter or area of a feature (using a tool, or with the measure tool for areas), it will use 2D Cartesian math, so this is not that accurate.

Particularly with areas, you should project the data to a projected coordinate system that's using an equal area projection like Albers or Lambert azimuthal.

Buffers are even more fun. The Buffer Tool is also using 2D Cartesian math, so you should definitely project it first if you go that route.

There's also a Project Wizard that you can add back into ArcMap. See:

http://support.esri.com/index.cfm?fa=knowledgebase.techArticles.articleShow&d=27818

You can change how it calculates the buffer. Normally, it fits an oblique Mercator projection to the features, but you can change it to use the data's coordinate system, the map's, or calculate a custom projection for each feature. See:

http://support.esri.com/index.cfm?fa=knowledgebase.techarticles.articleShow&d=17560

Melita 
  Melita Kennedy
ESRI Product Specialist