Converting datas to survex input

[Olly Betts]

On Mon, Nov 12, 2001 at 02:06:56PM +0100, Jerome Alet wrote:
> I'm planning to do some tests with cavern, and I've got a lot of fixed
> points but I've only got their Lambert coordinates (Lambert coordinates
> are used in France). What I've got is (x,y) Lambert(III) and altitude in
> metres. Does someone know if Lambert coordinates can be used directly with
> cavern or if I must convert them to another coordinates system ? 

Survex doesn't assume anything about the coordinate system being used,
except that it's perpendicular.  This is probably very close to being true
for all coordinate systems over the sort of area you're likely to be looking
at.

If your coordinates get very large you might get problems, but Survex can
cope with those produced by UTM zones - I'm not aware of a system which
produces larger numbers.

> I've tried to convert Lambert coordinates to longitude+latitude and then
> convert this to cartesian coordinates but I'm not sure if this is the
> right thing to do, because this conversion gives me new X and Y but also
> gives me a Z which is different from my altitude.

I'm not an expert in this area, but different coordinate systems can produce
different altitudes - I think it'll depend on the spheroid used in the
projection to approximate the curved shape of the Earth.

The pragmatic answer is that it's probably best to just use whichever
coordinate system seems is the natural one.  So if you have data in one
system, and want answers in the same one, use that one.

Cheers,
Olly