topic Re: Converting projected coordinates to polar coordinates in Graphics Programming

Converting projected coordinates to polar coordinates

Kishore_Kumar — Wed, 22 Mar 2017 14:45:33 GMT

Hi ,
I have a small query related to the output of %centroid macro. I need to know the units of the X-Y output from %centroid. Actually I want latitude & longitude of the centroid. For those reasons, I need to convert X-Y coordinates to polar coordinates. Do you have any idea how can I convert output form %centroid macro to polar coordinates?

Re: Converting projected coordinates to polar coordinates

ArtC — Fri, 24 Mar 2017 06:18:39 GMT

the conversion from cartesian to polar coordinate systems is an easy trig problem for SAS. I am not sure this is what you want, because i am not sure what %CENTROID produces, but the following will convert back and forth. Remember SAS uses radians as the measure of the angle so you may need to convert from degrees if you start at polar coords.

data have;
input x y;
datalines;
12 5
5 12
run;
data wantpolar;
   set have;
   r = sqrt(x**2 +y**2);
   alpha=atan(y/x); /* angle in radians */
   degrees = alpha*(180/constant('pi')); /* radians to degrees */
   * Polar to cartesian;
   x1 = r*cos(alpha);
   y1 = r*sin(alpha);
   run;
proc print data=wantpolar;
   run;

Re: Converting projected coordinates to polar coordinates

GraphGuy — Thu, 30 Mar 2017 17:20:07 GMT

I believe the centroid macro returns x/y centroids in whatever the units the x/y were in the original map.

Some of the SAS maps have long & lat variables, in addition to the x & y variables, that you would need to make use of ...

With the old traditional maps (in the maps library), such as maps.states, the x/y are projected values, and would not be much use to you. If you pre-process this map, and replace the projected x/y with the long/lat variables (which are in radians), then the centroid macro will return x/y in radians, and you could use the tip in the previous comment to convert to degrees.

With the new GfK maps (in the mapsgfk library), such as mapsgfk.us_states, the x/y are also projected values, and would not be much use to you either. If you pre-process this map, and replace the projected x/y with the long/lat variables (which are in degrees), then the centroid macro will return x/y in degrees.

Re: Converting projected coordinates to polar coordinates

Kishore_Kumar — Sat, 01 Apr 2017 20:05:27 GMT

Thanks everyone for replying back. The answers were really helpful. Thanks