[R-sig-Geo] Distance calculation

[Oritteropus]

Hi all, 

I intend to use gDistance function in rgeos package to measure the distance
among a large number of polygons (at various latitudes) in a vector map, to
a maximum distance of 1000 km in various directions.
It works rapidly compared to other methods but I have a few doubts about it:

1) Is the distance calculated corrected for the projection used? If not,
would the error be relevant at a scale of 1000 km? To what extent?
 Which projection would you suggest to reduce the error?

2) Is there any other function running in latlong and giving the corrected
geodesic distance?

3) My second problem is that gDistance calculate straight line distance,
however what I need is the distance through the dry lands. e.g. the distance
between a polygon in Italy and one in Greece should be calculated through
the dry lands, not through the sea. Is there any way to do this in a vector
function? Otherwise, should I use a different function combining vector and
raster maps?


Any suggestion would be greatly appreciated, thank you in advance!

Luca



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