[Proj] proj and Dutch RD

[Gerald I. Evenden]

On Wednesday 11 January 2006 11:17 am, Roger Oberholtzer wrote:
> On Wed, 2006-01-11 at 10:56 -0500, Gerald I. Evenden wrote:
> > On Wednesday 11 January 2006 09:31 am, Roger Oberholtzer wrote:
> > > I am trying to use proj on a Dutch project using 'RD'. I have
> > > encountered a problem.
> > >
> > > I am trying the command:
> > >
> > > 	cs2cs -r +proj=latlong +ellps=WGS84 +to +proj=stere
> > > +lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079
> > > +x_0=155000 +y_0=463000 +ellps=bessel +units=m +no_defs
> >
> > 	...
> > 	Seeing the "Double Stereographic" below I feel that you should be using
> > +proj=sterea rather than than the American or Snyder stere.
>
> If I use sterea, and set the input ellipse to bessel rather than wgs84,
> my calculated values are almost exactly the same (> 0.001 meters error).
> Yeah!

Given the following script:

vala=" +ellps=WGS84 +proj=stere \
+lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079 \
+x_0=155000 +y_0=463000"
valb=" +ellps=WGS84 +proj=sterea \
+lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079 \
+x_0=155000 +y_0=463000"
cat <<EOF >data
5.38763888888889 52.15616055555555 at the origin
5.88763888888889 52.65616055555555 1/2 degree from origin
6.38763888888889 53.15616055555555 1 degree from origin
EOF
echo "using Snyder Stereographic"
./lproj $vala data
echo "using Double Stereographic"
./lproj $valb data

where the test values are at the center of the projection and intervals of 1/2 
degree and 1 degree (in both latitude and longitude) from the origin.  The 
script generates the following output:

using Snyder Stereographic
155000.00       463000.00 at the origin
188832.98       518750.23 1/2 degree from origin
221895.96       574738.50 1 degree from origin
using Double Stereographic
155000.00       463000.00 at the origin
188832.02       518749.72 1/2 degree from origin
221892.14       574736.46 1 degree from origin

Note that there is no difference at the center or projection origin but the 
projections differ by several meters at a distance of 1 degree from the 
origin.

In your case, where you are datum shifting using the wrong but same  
projection in inverse/forward mode, you would probably produce minimal error.  
You could have probably used Transverse Mercator and come up with pretty much 
the same values.

What I am suggesting is that you are setting up a bad habit of referring to 
the incorrect projection for a certain area.

-- 
Jerry and the low riders:Daisy May and Joshua
"The being cannot be termed rational or virtuous,
who obeys any authority, but that of reason."
---Mary Wollstonecraft 1792