[mapserver-users] PHP Mapscript delivers map with larger extent than requested

[Chuck Jungmann]

I am using MapScript with PHP because I want to be able to request a map
with a url, change the content-type and send saveImage directly to
STDOUT.

An example (relative) URL:
map.php?lat=42.056542&lon=-88.035765&ext=300&pwide=500&ptall=500

Where "ext" is the width, in meters, on the ground (difference in
longitude), "ptall" and "pwide" are the pixel dimensions of the desired
map bitmap.  If my server returns the map image I request, I can have
the client calculate the coordinates of any pixel on the map image
without having to bother the server.

The server uses pwide/ptall and ext to determine difference in latitude,
then the corners of the rectangle for MapObj::setextent().

Before I got your response, I was doing everything in lat/lon
coordinates, but something in your note made me realize that I should
use the native projection which is in meters for fewer calculations and
an easier set of numbers to check the output.

Here is the stripped-out code (functions merged, error-checking
removed):

// load a minimal map, add layer with specific map:
$map = ms_newMapObj("/mapdata/minmap.map");
$map->setProjection("init=epsg:26916");
$lay = ms_newLayerObj($map);
$lay->set("name", "tempRaster");
$lay->set("status", MS_ON);
$lay->set("type", MS_LAYER_RASTER);
$lay->set("data", "taxmap.tif");
$lay->setProjection("init=epsg:26916");

// projection objects for converting point
$projlatlon = ms_newprojectionobj("proj=latlong");
$proj26916 = ms_newprojectionobj("init=epsg:26916");

$pt = ms_newPointObj();
$pt->setXY($lon,$lat);
$pt->project($projlatlon, $proj26916);

$halfTall = $ext * $pwide / $ptall / 2;
$halfWide = $ext / 2;

$map->setextent($pt->x-$halfWide, $pt->y-$halfTall, $pt->x+$halfWide,
$pt->y+$halfTall);

Given the example above, the extent values I use are:
414145.897891, 4656422.994605, 414445.897891, 4656722.994605,
exactly 300 meters X and Y.

Immediately after $map->setextent(), I look at the values of
$map->extent and the values are:
414095.793507, 4656422.994605, 414496.002275, 4656722.994605
the differences are 400.21 X, 300 Y.  4/3 is familiar as screen ratio,
but I don't see why that would apply here.

Here is the .tfw file for the map (confirming square pixels):
0.205001999999928690
0.0
0.0
-0.205001999999928690
413698.77641943324
4656861.2550850855

Thanks for your response.  I apologize for not including all this
information in my first note, but too many times I have spent a lot of
time writing a note like this only to get no responses.

Chuck Jungmann





My MapServer version info:
MapServer version 5.0.3 OUTPUT=GIF OUTPUT=PNG OUTPUT=JPEG OUTPUT=WBMP
OUTPUT=SVG SUPPORTS=PROJ SUPPORTS=AGG SUPPORTS=FREETYPE
SUPPORTS=WMS_SERVER SUPPORTS=WMS_CLIENT SUPPORTS=WFS_SERVER
SUPPORTS=WFS_CLIENT SUPPORTS=WCS_SERVER SUPPORTS=FASTCGI
SUPPORTS=THREADS SUPPORTS=GEOS INPUT=EPPL7 INPUT=POSTGIS INPUT=OGR
INPUT=GDAL INPUT=SHAPEFILE



On Sun, 2009-10-25 at 01:06 -0400, Stephen Woodbridge wrote:
> Chuck Jungmann wrote:
> > I am a new user of MapServer, using version 5.0.3 from the Ubuntu
> > repositories.  I attempted to compile everything from source, had some
> > trouble, then decided to use the repository shortcut at least until I
> > understood how the application works.
> > 
> > I am writing an application where the browser requests a map by setting
> > the src attribute of an img object.  Identifying locations on the
> > resulting map requires that the application knows the extent.
> > 
> > I calculate the extent of the map from various values, and use the same
> > algorithms on the server so the browser and the server agree on the
> > dimensions without having to negotiate.  However, when I request the map
> > from MapServer, the extent of the resulting map is always larger (both x
> > and y dimensions) than what I requested, although still centered on the
> > same point.
> > 
> > Is this behavior normal (I don't want to beat my head against the wall
> > searching for an irrelevant math error)?  Is there some setting that
> > might cause MapServer to more closely match my requested extent?  Could
> > this be an error that was fixed in a more recent version of MapServer?
> > 
> > Any words of wisdom would be appreciated.
> 
> Chuck,
> 
> There are a number of conditions that may make mapserver checnge the 
> extents requested.
> 
> 1) mapserver assumes square pixels, so if you ask for an extents that 
> has a different aspect ratio than that of you image size then mapserver 
> will make adjusts.
> 
> 2) if you ask for extents and a scale, I think, mapserver will use the 
> extents to determine the center and then adjust the extents to the scale
> 
> Are you using the CGI interface? what does you request look like? You 
> need to give us more concrete examples that we can try here to test out 
> and explain what is happening. What version of mapserver are you using?
> 
> /path/to/mapserv -v
> 
> Will report how it was build with options and version number.
> 
> -Steve W.
>