Geographically square pixels

[Ed McNierney]

Peter -

First, please always reply to the entire list.  The first person to reply may not be able to answer all your questions, and answering one question isn't a commitment to answer them ALL <g>.

I'm having a little trouble parsing your comments - what exactly are you trying to do?  It sounds like you're trying to compute the scale in feet or meters from the extent in "degrees", and if that's the case you need to also take into account the angle of the measurement - in case the measurement is neither due east-west or north-south.

Are you trying to display a scale bar, compute a distance between two points, or somehow reproject or rescale the source map image?

        - 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: peter Willis [mailto:peterw at borstad.com]
Sent: Wednesday, November 10, 2004 1:43 PM
To: Ed McNierney
Subject: Re: [UMN_MAPSERVER-USERS] Geographically square pixels

Hello Ed,

So, to define the pixel scale, the 'scale' value for the raster layer is the thing I set?
As you imply, this won't do me much good if the raster covers a substantial distance in the north/south direction. I was hoping for 4-corners + center-coordinate input to re-map based on the changing east-west image pixel size, as the swath travels south to north.....

Peter


Ed McNierney wrote:

>Peter -
>
>If you have an image in that projection, the scale MUST be expressed in "degrees".  That's not a very useful value because a degree of latitude isn't the same size as a degree of longitude, and "one degree" measured at an angle isn't a very useful measurement to most people.  However, that's the property of the map projection you're using, and that's what the scale will be.
>
>If you need to provide the ability to measure distances in feet or meters or the like, you will need to construct a JavaScript measurement tool or similar doodad to do the calculation.  The USGS does something like this on the National Map viewer site.
>
>       - 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: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU]
>On Behalf Of pw
>Sent: Wednesday, November 10, 2004 1:08 PM
>To: MAPSERVER-USERS at LISTS.UMN.EDU
>Subject: [UMN_MAPSERVER-USERS] Geographically square pixels
>
>Hello,
>
>In my configuration file (.map file)  if  I have an image that is
>projected with equal area degrees pixels,
>(ie: each pixel is evenly sized in 'seconds' not feet or meters) how can I set up mapserver to know what scale the pixels are in the X and Y direction for the raster?
>
>I have been looking at the proj4 manual and at the mapserver manual and there doesn't apper to be any way to define the extents of an input raster in degrees so that it can be mapped to, say, lambert.
>
>Thanks for any insight,
>
>Peter
>
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