[Proj] Pixel area errors

[Ken Mankoff]

Dear List,

I've just learned that GRASS has a function to provide the true scale for a given cell. From this, I can determine the map projection error. 

My understanding is that for a given projection, the "lat_ts" term of the proj4 string describing the projection is where 1 m in GRASS is actually 1 m. Elsewhere, errors grow. I'd like to track these.

I asked the following on the GRASS list and was advised that this list might be a better place to ask:

Is the scale error only in longitude and never in latitude? Or can it be in both? Is there a way to find the error for a length error rather than an area error? Specifically, for a vector line with an arbitrary heading?

Thank you,

  -k.