Ron,

You're proposing a piecewise-integration along a grid line. With that,

you'd also have to include the correction for scale factor at a point, as

integrated by so many points of choice along the projected line. Scale

factor along the Central Line of an Oblique Mercator is not going to

obviate that need.

It's far simpler to just call the subroutine in PROJ4 for an ellipsoidal

geodesic between the two end points. (Once called the "Principal Problem

of Geodesy" in the 19th Century).

Assuming you know how to program the subroutine calls, it's easier done

than said.

Clifford J. Mugnier

Chief of Geodesy and

Associate Director,

CENTER FOR GEOINFORMATICS

Department of Civil Engineering

LOUISIANA STATE UNIVERSITY

Baton Rouge, LA 70803

Voice and Facsimile: (225) 578-8536 [Academic]

Voice and Facsimile: (225) 578-4474 [Research]

================================

http://www.ASPRS.org/resources/GRIDShttp://www.cee.lsu.edu/facultyStaff/mugnier/index.html================================

What about using the Lat and Long of the endpoints to define the central

line of an Oblique Mercator, with a scale factor of 1.0 on the central

line?

Then the distance can be calculated by Pythagoras, and other useful

operations can easily be calculated - mid point, distance of a third point

from the line and even the calculation of a buffer zone, all of which seem

horrendous when working on the ellipsoid. (OK, the mid point is not too

bad).

Ron Russell

Tel : 01823 270308 email :

[hidden email]
-----Original Message-----

From:

[hidden email]
[mailto:

[hidden email]] On Behalf Of Clifford J Mugnier

Sent: 17 October 2005 20:52

To: PROJ.4 and general Projections Discussions

Subject: Re: [Proj] Mercator Problem

The Normal Aspect of the Mercator projection has only one practical

application, and that is for computing a Loxodrome or Rhumb Line with the

associated geometric manipulations/intersections of that line with the

graticule, etc. Computing distances is a folly on the Normal Aspect

Mercator. With two endpoints, just compute the inverse of the cartesian

coordinates back into latitude and longitude and then compute the correct

distance with a geodesic inverse.

I do not know how to perform that with PROJ4 commands.

Clifford J. Mugnier

Chief of Geodesy and

Associate Director,

CENTER FOR GEOINFORMATICS

Department of Civil Engineering

LOUISIANA STATE UNIVERSITY

Baton Rouge, LA 70803

Voice and Facsimile: (225) 578-8536 [Academic]

Voice and Facsimile: (225) 578-4474 [Research]

================================

http://www.ASPRS.org/resources/GRIDShttp://www.cee.lsu.edu/facultyStaff/mugnier/index.html================================

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@maps proj-4.4.9]# ./configure

[root@maps proj-4.4.9]# make

[root@maps proj-4.4.9]# make install

[root@maps proj-4.4.9]# cp ~/epsg /usr/local/share/proj/epsg

for GDAL

[root@maps gdal-1.2.6]# make clean

[root@maps gdal-1.2.6]# ./configure --with-ogr --without-python

--with-xerces

[root@maps gdal-1.2.6]# make

[root@maps gdal-1.2.6]# make install

[root@maps gdal-1.2.6]# /sbin/ldconfig

for mapserver

[root@tuna mapserver4.4.2]# rm -f config.cache

[root@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@tuna mapserver4.4.2]# make clean

[root@tuna mapserver4.4.2]# make

[root@tuna mapserver4.4.2]# cp mapserv /var/www/cgi-bin/mapserv_40

[root@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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