[PROJ] Correcting map projection errors

[Ken Mankoff]

Hi Thomas,

Thanks for the reply.

> But note that the figures you obtain are valid for infinitesimal items
> only. For large lengths/areas you should do the computations with
> geodesics on the ellipsoid

Can you clarify what you mean by this? Or more specifically, am I doing it incorrectly? I'm exporting a very large grid but at 200 x 200 m resolution to a CSV file, then using proj to calculate the error every 200 m. Is that close enough to infinitesimal? Or define "large lengths" please. I'm then using the errors to correct linear features at the ~200 m cell resolution.

> using e.g. Charles Karney’s Planimeter tool over at
> https://geographiclib.sourceforge.io/cgi-bin/Planimeter (online
> service, includes link to source etc.)

I've downloaded and compiled the code. Planimeter takes input in lat/lon, UTM/UPS, or MGRS. I'm working in EPSG:3413. I can provide lon,lat at each cell rather than x,y but I'm not sure if this introduces new errors.

> Also, a day where you have too much time on your hand, see the
> discussion over at
> http://osgeo-org.1560.x6.nabble.com/Re-Qgis-user-New-Features-in-Shape-Tools-3-2-0-td5378898.html

A nice read - not that I understood all of it - and one reason I use and contribute to OSS/FS.

  -k.