[R-sig-Geo] spatial modelling projection question

[Rich Shepard]

On Tue, 13 May 2014, Dominik Schneider wrote:

> Most people suggest longlat is not  a proper 'projection' for
> geostatistics ...

Dominik,

   longlat represent geographic coordinates, not a projection of
3-dimensional points on Earth to a 2-dimensional representation on paper or
a computer monitor.

> The standard projection for a domain my size/geographic location seems to
> be the conus albers from USGS (epsg:5070) which is an equal area
> projection so my question is: Wouldn't it make more sense to do spatial
> modeling with a true distance projection, i.e. longlat, than an equal area
> projection for which distances are skewed?

   What question(s) are you trying to answer with your data? Depending on the
size of the area analyzed you might find that UTM or State Plane Feet are
better projections for your use.

> What makes the variogram model potentially inappropriate on a sphere
> (overlooking the fact that the earth isn't really a sphere)? I appreciate
> your help understanding this.

   Every datum (e.g., NAD83 or NAD27; the North American Datums calculated in
the noted year) has errors because the Earth is neither a sphere or a
smooth ellipsoid.

   I highly recommend your studying Snyder, J.P. 1987. Map projections -- A
Working Manual. USGS Professional Paper 1395. It went out of print in the
early 1990s but is considered the benchmark for topographic map projections.
You can also read the documentation for Proj4, but John Snyder's monograph
will greatly increase your understanding.

   Understanding projections will help you select the most appropriate one
for each question you want to answer. You also need to be aware of what
happens when your analytical area is across two zones.

HTH,

Rich

-- 
Richard B. Shepard, Ph.D.        | Technically sound and legally defensible
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