[Proj] Mercator Problem

[tim]

Many thanks for all the feedback on this . . . after some poking around in 
the MapSever source code I think I found the reason for the measurement 
problems in MapServer with the Standard Mercator projection (most of the 
measurments for all projections are done with cartesian coordinates and not 
any kind of spherical or geoid calculations). If I am to find a solution it 
will be within MapServer.

thanks again
tim

Tim Norris

At 02:59 PM 10/17/2005, Craig Miller wrote:
>To get correct distances, you will need to calculated great-circle distance.
>
>--Craig
>
>
>-----Original Message-----
>From: tibben at ocf.berkeley.edu [mailto:tibben at ocf.berkeley.edu]
>Sent: Monday, October 17, 2005 12:36 PM
>To: proj at lists.maptools.org
>Subject: [Proj] Mercator Problem
>
>Perhaps this is not the place for this questions and I should be posting
>this observation to a different list . . . in any case input would be
>appreciated. Sorry in advance if this observation is out of place.
>
>Recently I reminded myself that measuring distance on maps is a difficult
>matter, and I have observed some errors regarding the distance measuring
>tools in mapServer (in particular with the use of the Mercator
>projection).
>
>After a little research, the problem regarding measuring distances when
>using the standard Mercator projection would be a variable issue based on
>lattitude. This is fairly obvious if we think about the way the Mercator
>projection works, and perhaps my best solution is to choose a better
>projection for my area of interest (California - perhaps a version of UTM
>would give me better measurments - in fact I am reasonably certain it
>would).
>
>But the big questions for me is - since there are errors within the
>distance measuring tools, do the same errors exist in the scalebar tools??
>
>After measuring a few scalebars and comparing them to paper maps of
>California it would seem that the answer is yes. All of the scalebars
>drawn through mapServer are short (using the Standard Mercator between
>lattitudes 33 and 38).
>
>Even though the answer is that there is an error using distances with the
>Standard Mercator with my system, I am, to say the least, unclear as to
>where this problem lies . . . PROJ4 problem?? . . . a compile problem??  .
>. . a GDAL problem?? some other issue???
>
>I am sure that I can fudge a workaround for this, but at the same time . .
>. any guidance for an eleant solution would be greatly appreciated . . .
>
>for reference the following were my build steps:
>
>for Proj4
>[root at maps proj-4.4.9]# ./configure
>[root at maps proj-4.4.9]# make
>[root at maps proj-4.4.9]# make install
>
>[root at maps proj-4.4.9]# cp ~/epsg /usr/local/share/proj/epsg
>
>for GDAL
>[root at maps gdal-1.2.6]# make clean
>[root at maps gdal-1.2.6]# ./configure  --with-ogr --without-python
>--with-xerces
>[root at maps gdal-1.2.6]# make
>[root at maps gdal-1.2.6]# make install
>[root at maps gdal-1.2.6]# /sbin/ldconfig
>
>for mapserver
>[root at tuna mapserver4.4.2]# rm -f config.cache
>[root at tuna mapserver4.4.2]# ./configure --without-tiff --with-threads
>--with-proj --with-gdal=/usr/local/bin/gdal-config --with-ogr
>--with-php=../php-5.0.4 --with-gd=/usr/local  --with-freetype=/usr/bin
>--with-pdf --with-wmsclient --with-wfs --with-wfsclient
>[root at tuna mapserver4.4.2]# make clean
>[root at tuna mapserver4.4.2]# make
>[root at tuna mapserver4.4.2]# cp mapserv /var/www/cgi-bin/mapserv_40
>[root at tuna mapserver4.4.2]# cp mapscript/php3/php_mapscript.so
>/usr/local/php/extensions/php_mapscript_40.so
>
>I am using mapServer with PHP mapscript on a Fedora/apache system and all
>my data is MapInfo (through OGR) - other than this problem, mapServer with
>PROJ4 has performed much better than commercial software that I have used
>in the past.
>
>thanks
>tim
>
>Tim Norris
>
>ps This has also been posted to the mapServer listserv.
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