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ArcPad NAD27 issues with GPS	Devlin Fung	Aug 05, 2003
Re: ArcPad NAD27 issues with GPS	Melita Kennedy	Aug 05, 2003
Re: ArcPad NAD27 issues with GPS	Devlin Fung	Aug 28, 2003
Re: ArcPad NAD27 issues with GPS	Jerry Grant	Sep 02, 2003
Re: ArcPad NAD27 issues with GPS	Matthew Bafford	Sep 02, 2003
Re: ArcPad NAD27 issues with GPS	Jerry Grant	Sep 03, 2003
Re: ArcPad NAD27 issues with GPS	Julie Luetzelschwab	Sep 03, 2003
Re: ArcPad NAD27 issues with GPS	Melita Kennedy	Sep 04, 2003
Re: ArcPad NAD27 issues with GPS	Julie Luetzelschwab	Sep 18, 2003
Re: ArcPad NAD27 issues with GPS	Mark Roper	Oct 02, 2003
Re: ArcPad NAD27 issues with GPS	Joel Cusick	Feb 08, 2004
Re: ArcPad NAD27 issues with GPS	Robin Poot	Aug 28, 2003
Re: ArcPad NAD27 issues with GPS	Chris Harvey	Sep 12, 2003
Re: ArcPad NAD27 issues with GPS	Julie Luetzelschwab	Oct 03, 2003
Re: ArcPad NAD27 issues with GPS	Evan Thoms	Oct 03, 2003
Re: ArcPad NAD27 issues with GPS	Mark Roper	Oct 03, 2003
Re: ArcPad NAD27 issues with GPS	Evan Thoms	Aug 05, 2005
Re: ArcPad NAD27 issues with GPS	Julie Luetzelschwab	Aug 08, 2005

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Subject	ArcPad NAD27 issues with GPS
Author	Devlin Fung
Date	Aug 05, 2003
Message	We have been working to implement a custom data collection solution utilizing a combination of ArcPad with GPS and ArcGIS 8.3. We have, however, run in to some problems with collecting GPS data in ArcPad using base layers that are in UTM Zone17N, NAD 1927. With the map in this projection, ArcPad incorrectly reprojects the incoming GPS coordinates into the map's projection. To be specific, the GPS coordinates appear (and are recorded) shifted by a fairly significant amount from the actual location. The offset is outside of the error that can be expected from normal GPS errors. Through testing with known locations,we have determined that ArcPad handles the GPS data correctly when the map is in UTM Zone 17N, NAD 1983. We have been unable to get accurate readings in NAD27 despite trying a number of different methods and settings. Our collection of GIS layers/images is significantly large enough that performing a datum conversion on all of our data is not an acceptable solution for the near term. Additionally, a significant portion of our user base still utilizes ArcView 3.2, and as such a mass datum conversion of our GIS data would cause compatibility problems with our users' legacy data and projects. As a temporary fix, until this bug with ArcPad is fixed, we have experimented with projecting our data in NAD83 for the purpose of data collection. Because a majority of our users are not GIS specialists, we need this process to be as transparent as possible. We were happy to discover that the ArcPad export wizard that is included with the ArcPad 6.x software will automatically project the output layers into the same spatial reference as the map. This wizard, however, has some fairly severe bugs which cause problems for application. For example, layers with complex geometries and layers with definition queries are not exported correctly. There are also issues with symbology and layer names when exporting using this version of the tool. ArcPad 8.3 ships with a new version of the ArcMap export wizard; this new version resolves the issues we had with the old version of the wizard. Our problem is that the 8.3 version of the wizard does NOT reproject the data. Essentially, we are left with two utilities for exporting to ArcPad, neither of which does what we need it to do. Our main issue is the problem that ArcPad has with GPS data and NAD27 map projection. Even if the export wizards did what we needed, they would still be a patch over the true problem, as we still be working in NAD 83 rather than being able to utilize our existing NAD27 data directly. We would appreciate if you could look into this issue, and provide us a solution to the problem we have encountered.

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Melita Kennedy
Date	Aug 05, 2003
Message	The first thing is to check which transformation is being used to convert from WGS84 to NAD27. You'll have to locate the apDatums.dbf file and see what the first NAD27 listing is. You can check the parameters against the Projection Engine pdf file that's available on the website. Go to http://support.esri.com and select Software, ArcInfo Desktop, White Papers. It's under 8.1.2. Or check on the NIMA website, http://164.214.2.59/GandG/datums/dtp/CountryNorthAmericaTable.html#NASC It might be that you're using a worse-than-normal transformation to convert to NAD27, or that you can switch to an 'eastern US' or 'western US' one and get better results. ArcPad only supports three and seven parameter datum transformation methods. Unless you can calculate a local seven parameter transformation for your data, none of the predefined transformations are really that good. The best transformation is the NADCON method which uses two files of shift values to model the differences between NAD27 and NAD83. The files are generally too large to support on hand-helds. You really should be using the NADCON method to convert the data. In general, NAD83 and WGS84 can be treated as if they are the same. Melita
	Melita Kennedy ESRI Product Specialist

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Devlin Fung
Date	Aug 28, 2003
Message	Melita, We ended up changing the apdatums file as you suggested to use a set of parameters that produce acceptable results throughout the area where we are using ArcPad. Thanks for your help, --Devlin

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Jerry Grant
Date	Sep 02, 2003
Message	Devlin, What changes did you make to ApDatums.dbf? Our GPS icon appears about 25m ENE of our topo, so we need to shift the UTMs reported by the GPS by x = -20 m and y = -15 m. Thanks for any help. Jerry

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Matthew Bafford
Date	Sep 02, 2003
Message	The calculations are not so simple that you can easily account for an x/y shift by just changing the dx/dy fields in apdatums. A change in one field can have siginificant impact on both the X&Y coordinates. What we ended up doing is something like this: From the datums table (http://164.214.2.59/GandG/datums/dtp/CountryNorthAmericaTable.html#NASC) we found the appropriate range of x,y,z values for our area. In our case, x: -9[+-5], y: 161[+-5], z: 179[+-8]. We then wrote an ArcGIS VBA program to loop over the range of values and perform a geocentric transform for each of the possible integer values in those parameter ranges. For input to the transform we passed in a known NAD83 monument's LL coordinates, and compared the output to that same monument's NAD27 LL coordinates. We ended up using about 40 monuments spaced throughout our target area. The parameter set that gave us the best match for all of those 40 monuments is what we ended up going with. We then modified the apdatums file and removed all of the NAD 1927 lines EXCEPT for one. On the remaining line we replaced the dx, dy, dz values with the values we obtained using the above method. Finally, we tested the actual ArcPad & GPS combination with the apdatums file we generated. Each monument sampled in this manner resulted in a point within 1m of the true coordinates. For our purposes, this is acceptable. There may be a better way to obtain the parameter set - if so, we'd love to hear about it. Sorry, but I don't have any code easily accesible to include. Cheers, --Matthew

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Jerry Grant
Date	Sep 03, 2003
Message	Thank you, I'll try that out. Jerry

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Julie Luetzelschwab
Date	Sep 03, 2003
Message	I'm so grateful to find your email. I work for the US Forest Service and all of our data is in NAD27. My position was generally over 200 feet shifted to the east using NAD27, so I tried switching to NAD83 and WGS84. It is still shifted about 3 meters to the east, but at least that is better. Still, this is entering more error into my data that I am trying to mask out with the PDOP setting. Trimble seems to deal with this without a problem... Can't ESRI fix this?

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Melita Kennedy
Date	Sep 04, 2003
Message	You need to see if the transformation that the Trimble unit is using is supported by ArcPad. And if it is, is it the first entry in the apDatums.dbf file? There are at least 20 different transformations that convert between NAD27 and WGS84. They're all designed for different areas. Please check out my reply earlier in this thread for more information. Melita
	Melita Kennedy ESRI Product Specialist

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Julie Luetzelschwab
Date	Sep 18, 2003
Message	I'm still confused by this issue. In ArcPad under OPTIONS I have set the datum to WGS84 because as I understand it, this is the datum your GPS unit uses, NOT your GIS layer's datum. My GIS layers are in NAD27. My GPS position seems to line up correctly with my NAD27 layers on the map. I create a line shapefile in ArcPad and it creates a line from the GPS positions as I walk. When I get back to the office and copy this shapefile to my PC. Its defined projection (.prj) matches my GIS data (UTM NAD27). I also had a Trimble ProXRS and corrected that file and projected it to UTM NAD27 with Pathfinder and saved it as a shapefile. I also had a Garmin eMap and I used DNR Garmin to convert the track to a shapefile in UTM NAD27. The Trimble and Garmin lines match up almost perfectly, but the ArcPad line is consistently shifted 17 meters. Is it the first NAD27 line in the apDatum.dbf file that ArcPad uses to transform my new shapefile from Geo WGS84 into UTM NAD27?

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Mark Roper
Date	Oct 02, 2003
Message	I too, work for the US Forest Service and I believe it is the same issue that ArcMap has with Project-on-the-fly WGS84 to a NAD27 datum. ESRI suggested that the San Juan NF set the ArcMap DataFrame transformation method for WGS84 to NAD27 to NAD_1927_to_WGS_1984_6 for our area. This method has a ID of 8075. This puts it to within about 1 meter of the true location. PE_GT_NAD_1927_TO_WGS_1984_5 8074 /* Mean for United States(CONUS East of Mississippi River including MN, MO, and LA) PE_GT_NAD_1927_TO_WGS_1984_6 8075 /* Mean for United States (CONUS West of Mississippi River) The first time the NAD27 projection is set in ArcPad, the default transformation for WGS84 to NAD27 is #1530 (top of list), which is the reason I was 12-15 meters off to the northwest. I was using a Trimble GeoXM for my test with both ArcPad plus GPScorrect as well as Terrasync software. In ArcPad, I set the transformation to #8075 and after post-processing with GPScorrect, I was within about 2 meters of the Terrasync point at the same location (a road intersection). Uncorrected (real-time), the point was about 2.5 meters north. Interestingly, when the datum method was set to #8074 (east US)the real-time location visually looked good until it was post-processed, putting it about 5 meters too far south. Depending on where you live, if you are using ArcPad and NAD27 for gps data, you may need to set the datum transformation to either #8075 (US west of Mississippi R.) of #8074 (US east of Mississippi R.) in order to get the best gps shapefile possible. If not, you will institute an error greater than that of the suggested accuracy of the gps receiver. Check ESRI's website if you need a list of all the datums. You should definitely test these datum methods for your area prior to collecting data for a project.

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Joel Cusick
Date	Feb 08, 2004
Message	Melita, Any idea when ArcPad will support NADCON strength Datum transform on handhelds? As I read this interesting thread, I can conclude two ways out of this Tarpit: 1) Allow NADCON 27-83 transform for those stuck in NAD27 GIS when using GPS (this assumes WGS84=NAD83 - a valid assumption for worldwide GIS applications). 2) We, the user community, tough it up and move to NAD83. The former could be done as a service to the number of users stuck in 27, until we shift over. NADCON can be loaded into handhelds (Trimble's TerraSync allows NADCON as does Solo Field). Joel Cusick

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Robin Poot
Date	Aug 28, 2003
Message	Your problem is the nomenclature. NAD27 is a datum and cannot be "reprojected," although it can be converted with a number of processes (such as the NADCON as suggested) with varying degrees of accuracy. Remember, the datum IS the coordinate. The projection is a representation of it. There is up to 30 metres (100') of distortion in NAD27 -- which will directly show up when using a 7 paramater similarity transform. Regards, Robin Poot

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Chris Harvey
Date	Sep 12, 2003
Message	We also work with NAD 27 UTM 17N and have discovered approx. 30m shift in our data when we project to a projection with a NAD 83 datum. However, we have come to a different conclusion about the cause of this. We use Arc Toolbox and the NADCON transformation to do our projecting, and our USGS base layer tiffs are a product of MapTech Terrain Navigator software. One of the GIS Specialists here, did an experiment where she converted the NAD 27 UTM TIFF into a grid, she then projected it using ArcInfo into a projection using a NAD 83 datum. The shift went away. The Terrain Navigator software does not afford the opportunity to select some of the parameters, (it seems to automatically assign a spheroid). We think that this may be the source of our shift issue.

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Julie Luetzelschwab
Date	Oct 03, 2003
Message	Mark Roper's response is very helpful to help find the appropriate datum for you area and Melita's link http://164.214.2.59/GandG/datums/dtp/CountryNorthAmericaTable.html#NASC also. So I found 8075 (-8, 161, 175) to work well in New Mexico so I moved this one to the top of the .../ArcPad/Systems/apDatum.dbf list so it is now my default. Or it can be selected by clicking on the LAYERS button and clicking on the DATUM button (world with 2 arrows). Everything lines up much better now!

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Evan Thoms
Date	Oct 03, 2003
Message	Ok, this is all very interesting, but what if, as in my case, you have already collected GPS points with a less than ideal transformation? How do you get them corrected? What I tried was to re-project them back to geographic coordinates based on WGS84 using what I thought was probably the default transformation being used when the points were collected. I then re-projected to NAD27 using the transformation that I think I should have used in the first place. The points came out worse than what I had collected. Is my method sound, but I picked the wrong transformations? It's getting to the point that I may simply move all of my points by dx and dy and call it good.
	Evan Thoms US Geological Survey Alaska Science Center (907)786-7409 ethoms@usgs.gov

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Mark Roper
Date	Oct 03, 2003
Message	You have the right idea. I was facing this very issue myself and (after examing projections in ArcGIS and Trimble's Pathfinder Office) I found you CAN reproject the data back into WGS84, then into the projection you really desire. I am assuming that you, like many of us, unknowningly used the 1530 datum transformation that was originally the default in ArcPad. The problem is that I have not found the datum transformation POSC code 1530 listed in any of the other programs or datum lists I have access to, so I had to treat it like a custom datum transformation. Now, in order to do this you need to know that when using 1530, ArcPad data going from WGS84 TO NAD27 is transformed using the following numbers: Transform X (m) 4 Transform Y (m) -135 Transform Z (m)-181 This matches what the apdatums file shows. In order for this shifted data to go from NAD27 TO WGS84, you will need to reproject using the following numbers: Transform X (m) -4 Transform Y (m) 135 Transform Z (m) 181 Positive and Negative numbers are reversed depending on which direction you go. Here is what I did. I used ArcToolbox for Arc8.1.2 and reprojected my ArcPad NAD27 shapefile back into WGS84 using the New Geographic Transformation option (when it asked for the transformation) and hand entered the -4, 135, 181 numbers. Once I had a WGS84 shapefile, I reprojected it into the correct NAD27 datum for my area. The shapefile lined up where I expected it to. If you don't have ArcToolbox, other options might be ArcInfo Workstation or Pathfinder Office. If ArcView Projection Utility has a way to customize/add datum transformations, I could not find it. If all else fails, drag your data to where you think it should go. I hope this helps.

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Evan Thoms
Date	Aug 05, 2005
Message	If you have been having trouble getting to the NIMA table of 3 parameter datum transformations, the new link is http://earth-info.nga.mil/GandG/coordsys/onlinedatum/CountryNorthAmericaTable.html#NASD and if you back up to: http://earth-info.nga.mil/GandG/coordsys/onlinedatum/index.htm you can search for a method interactively. Evan
	Evan Thoms US Geological Survey Alaska Science Center (907)786-7409 ethoms@usgs.gov

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Subject	Re: ArcPad NAD27 issues with GPS
Author	Julie Luetzelschwab
Date	Aug 08, 2005
Message	Hello, The fix you need is in this thread. You need to change the apdatums.dbf file so it uses the appropriate WGS84 to NAD27 conversion. The default one it uses is for the Carribean, which results in a significant shift of your GPSd data. In the apdatums.dbf file you need to move the one for your area so it is the first D_North_American_1927 in the list. East of the Mississippi is 8074, west of the Mississippi is 8075.