[Proj] Algorithms & Plate Carrée

[Irwin Scollar]

Roughly what order of magnitude of error would there be in N-S and 
E-W in coordinates compared to WGS84 UTM in a Google Earth image 
showing a 1 kilometer square at middle latitudes and close to the 
central UTM meridian, assuming no distortion due to other causes in 
the Google Earth image?

Is there a simple way to calculate this for arbitrary lat/lons 
between 70 degrees North or South like a Tissot Indicatrice for small areas?

Irwin Scollar