[ka-Map-users] equidistant projection assumption

[Moran Ben-David]

Thanks Paul.  I'll definitely check that out.  I was going to start looking
at proj4's code for Milller (which is the proj I'm using)... but as well as
that's written, it will be an easier job to use mapbuilder's code if they
support Miller.

Thanks again,
Moran

> -----Original Message-----
> From: Paul Spencer [mailto:pspencer at dmsolutions.ca]
> Sent: Tuesday, January 31, 2006 9:04 PM
> To: Moran Ben-David
> Cc: 'Daniel Morissette'; ka-map-users at lists.maptools.org
> Subject: Re: [ka-Map-users] equidistant projection assumption
> 
> Right, that's not really clear in the code.  ka-Map does assume that
> there is a proportional relationship between pixel and geographic
> space, but it doesn't assume a geographic projection ... as Daniel
> pointed out, MapServer deals with that.  However, ka-Map doesn't
> provide any sort of support for working in a different projection
> than the map.  For instance, you cannot zoom to a latitude and
> longitude using decimal degrees if your map projection is in LCC
> using meters (for instance).  The community mapbuilder project has a
> projection library in javascript that can do client-side reprojection
> in some cases that could be used.  Alternately you could hit a server-
> side script to reproject but that would be pretty slow I think :)
> 
> Cheers
> 
> Paul
> 
> 
> 
> 
> On 31-Jan-06, at 6:38 PM, Moran Ben-David wrote:
> 
> > Thanks, Daniel.  That was very helpful!  My initial issue/question
> > was due
> > to my confusion about projections as you pointed out.
> >
> > As you point out, I'll have to project my lat/lon (absolute) and
> > pass the
> > projected values to kamap's functions.
> >
> > Thanks,
> > Moran
> >
> >
> >> -----Original Message-----
> >> From: Daniel Morissette [mailto:dmorissette at dmsolutions.ca]
> >> Sent: Tuesday, January 31, 2006 5:02 PM
> >> To: Moran Ben-David
> >> Cc: ka-map-users at lists.maptools.org
> >> Subject: Re: [ka-Map-users] equidistant projection assumption
> >>
> >> 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/
> >> ------------------------------------------------------------
> >
> > _______________________________________________
> > ka-Map-users mailing list
> > ka-Map-users at lists.maptools.org
> > http://lists.maptools.org/mailman/listinfo/ka-map-users
> >
> 
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> |Paul Spencer                           pspencer at dmsolutions.ca   |
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