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UTM48 and GPS coordiinates

February 17 2010 | 0 comments
Categories: Map Data

Still hanging in the with UTM 48N Indian. The problem is now the GPS coordinates were collected in LAT/LONG on a Garmen Orgeon device with the factory default settings of WGS84 and Mercator projection. I did the normal math, brought to empty map, assigned the original projection, then changed it to the Indian UTM 48 and the values are out there some place NOT on MY MAP. They did a dump from the GPS to Google Earth and they are about 35-50 feet not correct. But after my conversion to decimal degrees, they are really off. Does the error lie in the standard lat/long conversion to dec/deg? What would actually give me the values I need to get them on the dec./degree map?

Mapping Center Answer:

I think perhaps the problem lies in getting the data into WGS84 before you project it to the Indian 48N UTM coordinate system. It's a little hard to follow exactly how the data were converted from the GPS to the GIS but let's see what we can figure out.  First, the data were either collected in WGS84 or Mercator but not both.  Let's assume they were collected in WGS84.  In that case, they might have been collected in either degrees, minutes, seconds (DMS), decimal degrees (DD) or degrees decimal minutes (DDM).  If it is the first, then the conversion using "normal math" would be degrees + minutes/60 + (seconds/60)/60. If it is the second, the conversion is already done.  If it is the last, it is degrees + decimal minutes (there are no seconds to convert.)

If you correctly converted the data to WGS84, then projecting to the Indian 48N UTM coordinate system shoudl be straightforward.  You will want to use one of these two transformation methods:  Indian 1960 to WGS84 No. 3 (for Vietnam) or Indian 1960 to WGS84 No. 3 (for Vietnam near 16 degrees N).

You state that you assigned the original projection -- again we wonder what that was -- was it WGS84 or Mercator?  If you can figure that out and make sure that you have the proejction for the original data collection right, then you can use the ArcGIS projection engine to convert to the Indian 48N UTM coordinate system.

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