[Proj] Mercator Problem

[Paul Ramsey]

Tim,

For a Mercator map of all of California, there is no *right* scale  
bar.  The scale at the top will be different from the scale at the  
bottom.  This is true of any Mercator map, just actually visible on  
maps of larger areas.  So the scale bar Mapserver is generating is  
probably right for *one* part of the map (perhaps the top or bottom?)  
and the scale bar from your paper map is right for *one* part of the  
map (the middle, maybe? or maybe for right where it is?).

For interactive things, like desktop user interfaces, it is possible  
to quietly cheat.  The measuring tool in uDig, for example, projects  
back into geographic coordinates, and does length calculations on the  
spheroid, rather than naively returning the length in the projected  
plane.  In this way it is possible to measure distances in meters  
while looking at a map in geographics, or get correct distances from  
a map in mercator.

Paul

On 17-Oct-05, at 12:35 PM, tibben at ocf.berkeley.edu wrote:

> 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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