Returns the value at a specified coordinate on the surface.

surf_id—(long) A valid ID for a gridded surface.

xvalue—(float) An x-coordinate value on the surface.

yvalue—(float) A y-coordinate value on the surface.

result—(float) The value of the surface at the specified point.

Nulval—(float) Specifies a value to be substituted for an “undefined” point.

The value of surf_id is obtained from the Surf_id keyword in the GTGRID function or from the output of the GTINSTALLSURF function.

GTGETVALUE accepts either scalar or array values for the x and y coordinate points. You can use GTGETVALUE to draw a line along the top of a surface drawing. First, create a surface using GTGRID or GTINSTALLSURF. Then generate an array of z values for x, y points along the surface using GTGETVALUE. To do this, you can use the original x, y vectors from your GTGRID call, or use FINDGEN to generate new x and y vectors. Then, use the z values returned by GTGETVALUE to overplot a line on the surface plot of your grid.