[Mapserver-users] MapScript: map-Y coordinate is always wrong while map-X is right

[Sean Gillies]

Hi Bjoern,

I found the problem in your code.  See below.

On Tuesday, July 8, 2003, at 07:57  AM, Bjoern Platzen wrote:

> Hello List,
>
> maybe I'm just to blind, but I can't figure out, what is going wrong.
>
> I wrote a small Program using PythonMapscript.
>
> I'm calculating the following to get the Map-Coordinates from the
> Click-Coordinates (where clickX and clickY come as img.x and img.y from
> a HTML-form, Extend is the [mapext]):
>
> def _click2map(self,clickX, clickY, Extend):
>     '''Uebersetze Klick-Koordinaten in Kartenkoordinaten'''
>     Extend = Extend.split(" ")
>
>     cx = (float(Extend[2])-float(Extend[0]))/(self.mymap.width-1)
>     cy = (float(Extend[3])-float(Extend[1]))/(self.mymap.height-1)
>
>     xMap = float(currentExtend[0])+cx*clickX
>     yMap = float(currentExtend[1])+cy*clickY
>

should be:

       yMap = float(currentExtent[1])-cy*clickY

Remember that the origin of your image is the upper left corner
and so your new map point will always have a smaller y value.  Also,
it looks like you are calculating cx and cy incorrectly.

Here is an example of how this calculation is done in ZMapServer
(http://zmapserver.sourceforge.net):


         extent = (mapobj.extent.minx,
                   mapobj.extent.miny,
                   mapobj.extent.maxx,
                   mapobj.extent.maxy )
         width = mapobj.width
         height = mapobj.height
         pxsize = (extent[2]-extent[0])/width
         pysize = (extent[3]-extent[1])/height
         point = {}
         point['x'] = extent[0] + pixel*pxsize
         point['y'] = extent[3] - line*pysize

cheers,
Sean

--
Sean Gillies
sgillies at frii dot com
http://www.frii.com/~sgillies