[ka-Map-users] equidistant projection assumption

[Daniel Morissette]

The short answer is that whatever projection your data projected to, the 
projected coordinates, usually in meters or feet relative to the 
projection's origin, are in a cartesian system, i.e. the x and y axis in 
projected coordinates are linear, and it is safe to use this kind of 
maths to convert from projected coordinates to pixels and back.

This being said, the relationship between geographical coordinates (lat 
and lon, the absolute/unprojected reference ssytem) and the projected 
coordinates (x and y in meters, feet, whaetver) is not linear... but 
MapServer deals with that part inside the mapfile and ka-Map doesn't 
need to worry about it, unless you would like to offer multiple 
projection choices to your users in the user interface... in which case 
you would probably be better with Chameleon or another more gis-ish 
webmapping framework than with ka-Map.

I hope that helps

Daniel

Moran Ben-David wrote:
> Hi all,
> 
>  
> 
> I have been going through the kaMap code and noticed what I think is an 
> assumption that the projection of the map that it is using is 
> equidistant.  Am I right?
> 
>  
> 
> I am about to write my own version of various functions such as zoomTo 
> (where I noticed this assumption), however, I just want to make sure I’m 
> not wrong in this before I proceed.
> 
>  
> 
> Here’s a code snippet (kaMap.js) in zoomTo()
> 
>  
> 
> this.cellSize = newScale/(oMap.resolution * inchesPerUnit[oMap.units]);
> 
> …
> 
> var cpX = cgX / this.cellSize;
> 
> var cpY = cgY / this.cellSize;
> 
>  
> 
> The translation there from geo to pixel space seems to assume that y 
> (pixel) is proportional to y (geo) linearly… I think… am I off track 
> here entirely?
> 
>  
> 
> Thanks in advance,
> 
> Moran Ben-David
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> ka-Map-users mailing list
> ka-Map-users at lists.maptools.org
> http://lists.maptools.org/mailman/listinfo/ka-map-users


-- 
------------------------------------------------------------
  Daniel Morissette               dmorissette at dmsolutions.ca
  DM Solutions Group              http://www.dmsolutions.ca/
------------------------------------------------------------