I want to geo-reference a series of map, digitize features using polygons, and calculate accurate area measurements. I encountered the following issue:
After georeferencing a map image (.png) using the lat-long posted on the four corners of the map and projecting it using an appropriate UTM projection (WGS_1984_UTM_Zone_29N), I noticed that the scale bar is not accurate (the 50 km scale bar measures 34.4 km on the projected map). However, using the Measure Tool to find the length (in km) of 1 degree of latitde and lengths of 3.5 degrees longitude along both 46 deg. & 47.5 deg. latitude; the measurement results and corresponding errors are listed in the table below:
Note the large error from the scale bar (>30% error) and relatively insignificant error on the latitude and longitude measurements (<0.1% error).
1. Why would the scale bar length be inaccurate (especially when the image lat-long appear to be projected correctly and can be measured accurately in ArcMap)?
2. Is this an error in my geo-referencing? If so, is there a better method for geo-referencing this type of map image?
3. Am I using an incorrect projection? I believe a projected UTM is appropriate for area calculations and should provide accurate length measurement for features located within the UTM zone.
Any assistance is appreciated. Thanks!