[Mapserver-users] Using mapserv.js with decimal degrees

[Ed McNierney]

Matt -

Of course, you do have the option of simply reprojecting the MAP, rather
than the datasets.  You can leave your datasets untouched and have
MapServer produce the output map in whatever projection you choose.

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242  

-----Original Message-----
From: Matthew Perry [mailto:mtperry78 at sbcglobal.net] 
Sent: Tuesday, May 04, 2004 12:03 PM
To: Ed McNierney; mapserver-users at lists.gis.umn.edu
Cc: Gregor Mosheh
Subject: RE: [Mapserver-users] Using mapserv.js with decimal degrees

We have, for the time being, decided that we must use projected data
with mapserv.js. In order to get accurate scale calculations necessary
for the navigation tools, the units must be planar.

The solution might involve some javascript conversion of lat/long mouse
clicks and extents into planar distances, doing the scale calculations,
then converting back to lat/long. It would be much easier to simply
reproject the datasets.

Thanks to everyone for their help.

Matt Perry
Natural Resources Planning
Humboldt State University
mtperry78 at sbcglobal.net

--- Ed McNierney <ed at topozone.com> wrote:
> Matt -
> 
> The problem with unprojected data is that it's a strange and 
> hard-to-interpret data set when you try to measure something!
> 
> If your data is displayed in "decimal degrees" that means that one 
> degree of latitude and one degree of longitude form a square.  Of 
> course, they don't really do that anywhere on the Earth.  And unless 
> you're really close to the equator, one degree of longitude is 
> noticeably shorter than one degree of latitude.  And the length of one

> degree of longitude varies depending on the latitude.
> 
> So you've got a map that lets users measure "one degree" - what does 
> that mean?  It means that you're taking those angular units and trying

> to pretend they're linear units.  If someone measures between two 
> points on your map and measures one degree, they will know that the 
> square root of the sum of the latitude difference squared and the 
> longitude difference squared equals one.  It does not tell them 
> anything about how far apart those two points are in any useful linear

> measurement (feet, miles, meters, cubits, chains, furlongs, etc.).
> 
> And if you display a scalebar on that map, the units must be in 
> "degrees", whatever they are <g>.
> 
> What kind of measurement are you hoping to display?
> 
> 	- Ed
> 
> Ed McNierney
> President and Chief Mapmaker
> TopoZone.com / Maps a la carte, Inc.
> 73 Princeton Street, Suite 305
> North Chelmsford, MA  01863
> ed at topozone.com
> (978) 251-4242
> 
> -----Original Message-----
> From: Matthew Perry [mailto:mtperry78 at sbcglobal.net]
> 
> Sent: Monday, May 03, 2004 3:01 PM
> To: mapserver-users at lists.gis.umn.edu
> Cc: Gregor Mosheh
> Subject: [Mapserver-users] Using mapserv.js with decimal degrees
> 
> Hey all,
>   I have been using the mapserv.js and dbox.js libraries for 
> developing mapserver interfaces. It works great for projected data but

> now we're trying to get the navigation tools (panning, zooming, etc) 
> to work with decimal degrees.
> 
> In the mapserv.js, there is a global variable called InchesPerMapUnit 
> that you need to set correctly if not using meters.
> Since lat/long is
> measured in angular units (degrees), how would you set this variable?
> 
> Are there any other tricks to get the coordinate calculations working 
> with lat/long? Anyone had any luck with this? Is it even possible?
> 
> Thanks for any input!
> 
> Matt Perry
> Natural Resources Planning
> Humboldt State University
> mtperry78 at sbcglobal.net
> 
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