On Thu, 19 Jul 2012, Barry Rowlingson wrote:
On Thu, Jul 19, 2012 at 3:11 AM, Gregovich, Dave P (DFG)
<[email protected]> wrote:
Hello,
I apologize for reposting, but am still confounded after spending more time
with the following problem, despite some help from the group.
I am attempting to learn GIS in R. I have worked with ArcGIS a lot, and know
that my data is projecting correctly in that environment relative to other
trusted data. When I import a text file of animal locations into ArcGIS,
then define the projection as WGS84, then transform to NAD83, I can obtain
proj4strings from the resulting shapefiles to use as follows:
Yeah there's a constant shift between the R coordinates and the ESRI
coordinates:
> coordinates(bear_R_nad83)-coordinates(bear_ArcGIS_nad83)
coords.x1 coords.x2
[1,] 4181322 -5575097
[2,] 4181322 -5575097
[3,] 4181322 -5575097
- a workaround would just be to add this offset to your coordinates....
I've tried to figure out where this is coming from but with no
success. I've not had any problems with coordinates as long as I stick
to EPSG-defined coordinate systems. I don't have a copy of ArcGis to
hand, but I recall it has all sorts of other systems in it....
The EPSG codes 3468 and 26931 (Alaska I NAD83) are off by:
coordinates(bear_nad83) - coordinates(spTransform(bear_wgs84,
+ CRS("+init=epsg:26931")))
coords.x1 coords.x2
[1,] 1.191571 -0.4911515
[2,] 1.191572 -0.4911513
[3,] 1.191571 -0.4911517
....
with the main difference being:
CRS("+init=epsg:26931")
CRS arguments:
+init=epsg:26931 +proj=omerc +lat_0=57 +lonc=-133.6666666666667
+alpha=323.1301023611111 +k=0.9999 +x_0=5000000 +y_0=-5000000
+gamma=323.1301023611111 +datum=NAD83 +units=m +no_defs +ellps=GRS80
+towgs84=0,0,0
and
CRS(proj4string(bear_nad83))
CRS arguments:
+proj=omerc +lat_0=57 +lonc=-133.6666666666667
+alpha=-36.86989764583333 +k=0.9999 +x_0=5000000 +y_0=-5000000
+datum=NAD83 +units=m +no_defs +ellps=GRS80 +towgs84=0,0,0
+alpha being 360-alpha, and +gamma being declared. readOGR() is reading
the contents of bear_ArcGIS_nad83.prj correctly, +alpha is given there as
a negative number, and no +gamma is given, which appears to be why
projection is failing. There are more details of the complications of the
(Hotine) oblique mercator here:
http://www.remotesensing.org/geotiff/proj_list/epsg_om.html
Hope this helps,
Roger
Barry
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--
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: [email protected]
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