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 <rip...@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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PLEASE do read the posting guide
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--
Brian D. Ripley, rip...@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
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
