Fix real world cell size

[dodie]

The application reads HGT data. Each HGT file contains elevation data for a one degree latitude by one degree longitude tile in high resolution (E.g. N44E016).

This region is divided into 3600 rows and 3600 columns, each cell encoding the height data for a given location.

The real-world size of such cell depends on its latitude and longitude.

• Degrees of latitude are parallel, so the distance between each degree is mostly constant, 1 degree ~= 111km, so the height of each pixel is roughly 30.8 m.
• Longitude varies based on the actual location. It is also ~= 111km at the equator, but shrinks to 0 as it reaches the poles.

However, currently the application uses 30*30m for the size of all cells for simplification, which causes northern regions to be bigger than they are.

Fix this distortion with a better approximation.

Additional resources:

• SRTM User Guide
• Latitude on Wikipedia