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With increasing use of GPS coordinates in GIS's still stuck in NAD27, we see more datum translators on the market and a confusing array of coding schemes. I tested CORPSCON, ArcGIS (8.1), Trimble (Pathfinder Office 2.9) and ArcView 3.2a(Projection Utility Wizard) to see how these programs handle a translation of a WGS84 coordinate to NAD27 for Alaska Users.
An attachment (see.jpg) will assist in this disscussion.
If someone can confirm my suspicisions please do:
1) To retain the greatest accuracy of a GPS position, taking a raw WGS84 coordinate from a GPS is better translated to NAD27 by "lying to" ArcINFO/ArcView and Trimble that the incoming coordinate is NAD83 and then use NADCON translation to NAD27.
Test conducted thusly:
The attached JPEG shows a CORPSCON(uses NADCON) translated X at center and its relationship to other datum translated positions using other software.
In aggreement with the original point are: **Trimble translated (Assumed 1983) to NADCON27 AND **ArcGIS doubly translated: From WGS84 to NAD83, then from NAD83 to NAD27; **ArcView (lied to by converting from NAD83 to NAD27 using NADCON).
Located 0.9meters away is the same point converted using the 7 parameter Multiple Regression within Trimble.
Located 1.5meters away and all within very good agreeement with each other are the following: **ArcView's Projection Wizard WGS84-NAD27(POSC8076) converted point; **ArcGIS (ArcToolbox Projection Wizard-(WGS84-NAD27 Geocentric Translation POSC 8076); Trimbles (Export using NAD27Alaska and the Molendensky Translation ).
SUMMARY:
If using Mapping Resource GPS equipment under close scrutiny, additional errors may be introduced to your exported NAD27 GIS if you use the following:
1)ArcView's Projection Wizard or ARCGIS Geocentric translation using 3 parameter (1.5meters seen in Alaska at one location). Therefore, stay well away from any "NAD27_To_WGS84_*" Translations.
2)It does "pay" to cheat ArcView into thinking to use NAD83-NAD27 translation. Simply add a NAD83 PRJ file to all your incoming RAW WGS84 GPS data and use NADCON. This all of course assumes the difference between WGS84 and NAD83 is neglible.
3)Excellent agreement between Trimble, Corpscon, ArcView and ArcGIS(using double translations) for converting a WGS84 coordinate to NAD27 assuming a NAD83 input. This is comforting....
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