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I have gps data in WGS84 geographic coordinates. I will use the gps data to build a line shape in ArcMap 10.0. The data falls in UTM 15N.

To get most accurate length of line, should I project the point data to the UTM and then build the line in the UTM. Or should I build the line in WGS84 then project the line data to the UTM. Or does it not matter? What are the effects?

If UTM data doesn't best preserve distance then I will add that UTM must be used.

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You say your data is in WGS84. Is that Geographic WGS84? WGS84 is a datum, so your UTM data could be using WGS84 as well. –  Jeff Berry Jan 9 '12 at 17:40
    
Geographic WGS84 unprojected. –  Justin Jan 9 '12 at 17:57
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If you can build a track or route within the gps, then save it out as a polyline shapefile then project it in ArcMap, particularly if you need to perform a datum transformation (ie WGS84 to NAD83) along with the projection. If you are using a Garmin GPS unit, then the DNRGarmin extension can help dnr.state.mn.us/mis/gis/tools/arcview/extensions/DNRGarmin/… –  Dan Patterson Jan 9 '12 at 18:02
    
The data is sub centimeter trimble data. I can build the polyline in ArcMap from the points. My question seeks to find if there is a difference if I project the points then create the polyline in the projected space or create the polyline in WGS84 then project the polyline. –  Justin Jan 9 '12 at 18:15
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You mention sub-centimeter accuracy, so let's be careful. The answer depends on how far the point-to-point distances are and what latitudes are involved. At anything but extreme polar latitudes, and with distances less than 10 km or so, there will be no detectable difference (when measuring to the nearest millimeter). –  whuber Jan 9 '12 at 20:14
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1 Answer

If UTM is required as the output map projection then regardless of which map projection you generate line work in, they will be transformed once you project them to UTM.

UTM is pretty accurate for smaller linear measurements along the meridian. At the meridian it applies a scale factor of 0.9996. As you move east/west away from the meridian distortion will increase. If you are conserned about accuracy in linear measurments using UTM, then as you move away from the meridian it will become necessary to project that data into the next UTM zone, where it will be closer to that zones meridian.

So, I would project the data into UTM/WGS84, (assuming that's the final datum you want to use), and build my line work there.

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