[R-sig-Geo] [DKIM] Re: spatial modelling projection question [SEC=UNCLASSIFIED]

Wed May 14 03:01:31 CEST 2014

The effects of spatial reference systems on the predictive accuracy have been preliminarily tested recently. Please see 
W. Jiang, J. Li. 2013. Are Spatial Modelling Methods Sensitive to Spatial Reference Systems for Predicting Marine Environmental Variables? In: International Congress on Modelling and Simulation (MODSIM) 2013, Adelaide. Dec. 2013.
W. Jiang & J. Li. 2014. The effects of spatial reference systems on the predictive accuracy of spatial interpolation methods. Geoscience Australia, Record 2014/01, 33p.
They are freely available online. Any comments are appreciated!
Cheers,
Jin

-----Original Message-----
From: r-sig-geo-bounces at r-project.org [mailto:r-sig-geo-bounces at r-project.org] On Behalf Of Michael Sumner
Sent: Wednesday, 14 May 2014 10:36 AM
To: Rich Shepard
Cc: r-sig-Geo at r-project.org
Subject: [DKIM] Re: [R-sig-Geo] spatial modelling projection question

On Wed, May 14, 2014 at 4:53 AM, Rich Shepard <rshepard at 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 at gmail.com

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