On Wed, May 14, 2014 at 4:53 AM, Rich Shepard <

[hidden email]>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

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