On Wed, May 14, 2014 at 4:53 AM, Rich Shepard <rshepard@appl-ecosys.com>wrote:

> 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.
>
>

I believe that Dominik is referring to the fact that with longlat we can
use ellipsoid calculations for distance, which cover the most general
solution for "most accurate" across many situations. There are many map
projections that are more suitable for given applications, but then the
general confidence about Cartesian distances is gone: we need to be careful
that our distance calcs are appropriate within the constraints of that
projection. Any equi-distant projection can provide best accuracy only
within specific constraints.  (This is not true for area, those can be
always accurate in an equal-area projection albeit with loss of
conformality and usual numeric/topology constraints).

I think the only general solution is to back-transform and use the
ellipsoid for distance, unless you can be sure that the Cartesian methods
match your needs in the projection being used.  The only way to match
ellipsoid accuracy is to generate a local unique equi-distant projection
for each distance segment and measure from its centre to the other point.
(I think Manifold does this internally for the interactive
distance-measuring Tracker tool when you toggle the ellipsoid key. ).

I've been bothered by this for a while, especially with such a wide use of
UTM out there. I think you always need to be concerned about it, and do
some checks across your region/application to make sure Cartesian distance
is good enough when you have chosen a projection for other properties.  If
you can forget the need for expensive ellipsoid calculations or many local
equidistant projections it's obviously going to be more efficient.

Cheers, Mike.







>
>  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
> Applied Ecosystem Services, Inc. | ... guaranteed.
> www.appl-ecosys.com      Voice: 503-667-4517         Fax: 503-667-8863
>
>
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-- 
Michael Sumner
Software and Database Engineer
Australian Antarctic Division
Hobart, Australia
e-mail: mdsumner@gmail.com

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