[R] why variations in accuracy between R to ARCGIS for the same point reprojection?

[Camilo Mora]

I apologize, I did not intend to blame anyone. I actually thought  
there may be some underlying differences in the formulas used. The  
patterns overall are very similar but they are not that precise to one  
another. The function I use in r is called spTransform from the  
package "rgdal".

Again, my guess is that it has to do something with the functions  
used. As an example, I found two different ways to measure the radius  
of the Earth. So perhaps there is an additional parameter that  
differentiates the scripts from R and ArcGis?.

Sorry again for the confusion and thanks,

Camilo
p.s. I always pride R. It is just hard to image academic life without it...

Camilo Mora, Ph.D.
Department of Geography, University of Hawaii
Currently available in Colombia
Phone:   Country code: 57
          Provider code: 313
          Phone 776 2282
          From the USA or Canada you have to dial 011 57 313 776 2282
http://www.soc.hawaii.edu/mora/



Quoting Prof Brian Ripley <ripley at stats.ox.ac.uk>:

> There is no 'reprojection' in R (which is upper case).  Please  
> attribute blame correctly.
>
> You seem to be talking about some contributed addon package, not  
> specified.  But I think you should be asking this on R-sig-geo.
>
> On 02/09/2012 19:24, Camilo Mora wrote:
>> Hi everyone,
>>
>> I wonder if anyone knows the reason why the outputs of the same
>> reprojection in r and arcgis are different?. The magnitude of the change
>> can be up to 40 km in the poles.
>> Basically, I have a database of points equally separated by one degree
>> over the globe.
>> In ARCGIS,  I am projecting the data in GCS-WGS-1984 and then
>> reprojected it to Berhmann to ensure equal area distribution of the points.
>> In R, I am using:
>> spPoint <-
>> SpatialPoints(coords=coordinates(Data),proj4string=CRS("+proj=longlat
>> +datum=WGS84"))
>> and then reprojecting it to Berhmann with:
>> spPointReprj=spTransform(Data,CRS("+proj=cea +lon_0=0 +lat_ts=30 +x_0=0
>> +y_0=0 +ellps=WGS84"))
>>
>> If I put the two outputs of the reprojections in the same map, they are
>> off by few meters in the tropics by up to 40km in the poles.
>>
>> I decided to investigate how the reprojections are done and my
>> calculations are different from both R and ArcGis:
>>
>> First, I calculated the radious of the Earth in two different ways:
>> =Re * (1 - e^2)/ (1 - e^2 *SIN(RADIANS(Latitude))^2)^(3/2)
>> =Re * SQRT(1 - e^2) / (1 - e^2 * (SIN(RADIANS(Latitude)))^2)
>>
>> where Re is the radius of the Earth at the tropics(6378km) and e is the
>> eccentricity of the ellipsoid (0.081082).
>>
>> According to several books I used, the position of a point in the Y-axis
>> in the Berhmann projection could be estimated as:
>> =Re*(SIN(RADIANS(Latitude))/COS(RADIANS(Spll)))
>> where Spll is the standard parallel, which in the Berhmann's projection
>> is 30.
>> Unfortunately, the resulting output, although similar in shape to the
>> outputs in R and Arcgis, is still not quite the same. Any thoughts why
>> these differences in supposedly the same calculations?
>>
>> Any input will be greatly appreciated,
>>
>> Thanks,
>>
>> Camilo
>>
>>
>>
>>
>> Camilo Mora, Ph.D.
>> Department of Geography, University of Hawaii
>> Currently available in Colombia
>> Phone:   Country code: 57
>>          Provider code: 313
>>          Phone 776 2282
>>          From the USA or Canada you have to dial 011 57 313 776 2282
>> http://www.soc.hawaii.edu/mora/
>>
>> ______________________________________________
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>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
>
> -- 
> Brian D. Ripley,                  ripley at stats.ox.ac.uk
> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
> University of Oxford,             Tel:  +44 1865 272861 (self)
> 1 South Parks Road,                     +44 1865 272866 (PA)
> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>
>