imicola wrote:
Hi,
I read somewhere that when carrying out geostatistical analysis in R you
should not use latitude and longitude...can anyone expand on this a little
for me, and what would be the best coordinate system to use?
Not only in R. In most systems, the inter-point distances are assumed to
be planar (distances over an Euclidean space), whereas latitude and
longitude are spherical. I guess there could be a geostatistical
analysis based on spherical distances, but why to make things more
complicated when projecting the spherical coordinates into planar
coordinates b4 the analysis produces a good approximation and simplifies
the analysis significantly? I use UTM coordinates, and transform from
geodetic to metric with Eino Uikkanen's GeoConv program (it's free).
I have my data in a geographic coordinate system, WGS84, decimal
degrees....is this the wrong format for such analyses?
If the distances are short, it is not so wrong, and the wrongness
increases with increasing distance.
I have also converted my data in the UTM projection and so have it in
metres....(ranging from 480,000 to 550,000 E and 170,000 to 230,000 N).
If I was to use the UTM coordinates, should I be using the actual
coordinates in metres, or should I convert this into an arbitrary coordinate
system (i.e. from 0 - 1) somehow?
It would be convenient to use km rather than m, so the range parameter
would be closer in magnitude to the other parameters of the model. A
very large range parameter in metres may cause numerical instability
during minimization of the negative support function in likelihood-based
models such as that implemented in geoR.
I have noticed that running an analysis on the data gives different results
depending on which type of system you use, so I want to make sure I have the
correct system. I should also probably note that I am a geostatistical
novice!
Thanks,
Bottomline, your geostatistical software is probably based on distance
calculations on an Euclidean space so it is wrong to input locations in
spherical coordinates.
HTH
Ruben
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