Tim - I think you're confusing a few issues and only partially understanding the properties of the Mercator projection.
As Cliff pointed out, if you are interested in measuring distances, a Mercator projection is just the wrong choice of projection. Different projections have different properties; none are perfect, and all sacrifice doing some things poorly in order to do other things well. Again, Cliff pointed out the one prominent reason to choose a Mercator projection - it lets you draw lines of constant compass declination (rhumb lines) as straight lines. Unless you're a marine navigator with a defective GPS receiver, that is probably not useful to you. I have occasionally vented my frustration at the rampant proliferation of the "geographic" projection these days, in which latitude and longitude are used X/Y coordinates. This isn't the same as the Mercator projection, but it's very nearly as useless. You *cannot* create a generalized scale bar for such a map. That's not MapServer's fault, and has nothing to do with a MapServer configuration. You just cannot do it because the linear scale on such a map varies with both the latitude and with the angle of the line measured. It is just plain impossible to create an accurate scale bar. The fact that some maps display a scale bar next to a Mercator projection doesn't mean that scale bar is accurate (although the more honest ones will footnote the scale bar indicating the one place on the map where it's correct). Fortunately, there are many projections in the world to choose from <g>. The UTM family of projections was defined (in part) to provide relatively uniform distance measurements. That's why each UTM zone is only 6 degrees wide; the projection math works fine beyond that range, but the distortion increases to an unacceptable level. That's also why US states have adopted State Plane Coordinate Systems (SPCS). They are designed to cover a smaller area than UTM, but with a smaller degree of distortion, suitable for use in surveying and other professional applications. Many states are divided into several "zones", each of which uses a different (but related) projection, to further minimize the distortion. If you are specifically interested in the State of California, the State GIS program uses an Albers projection for managing statewide data. While nothing's perfect, this projection is probably a good candidate for use in displaying California statewide data sets in a manner that allows distance and area measurements (and scale bars) with a reasonable degree of accuracy over a relatively large area like California. You can find information about it at http://gis.ca.gov/albers.epl - Ed Ed McNierney President and Chief Mapmaker TopoZone.com / Maps a la carte, Inc. 73 Princeton Street, Suite 305 North Chelmsford, MA 01863 Phone: +1 (978) 251-4242 Fax: +1 (978) 251-1396 [EMAIL PROTECTED] -----Original Message----- From: UMN MapServer Users List [mailto:[EMAIL PROTECTED] On Behalf Of tim Sent: Monday, October 17, 2005 5:08 PM To: [email protected] Subject: Re: [UMN_MAPSERVER-USERS] Upgrade Problems - Additional Info Hi Frank, I appreciate your interest on this, but perhaps taking a closer look at the Mercator first would be more fruitful. I got a number of replies from the PROJ4 list that indicate the problem is related to the Mercator characteristics, i.e. From: Clifford J Mugnier <[EMAIL PROTECTED]> >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. From: Paul Ramsey <[EMAIL PROTECTED]> >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. From: "Craig Miller" <[EMAIL PROTECTED]> >To get correct distances, you will need to calculated great-circle distance. I am almost sure that I can implement one of these solutions through php mapscript, but it will mean circumventing the usual measuring tools . . . If you are still interested, I will try to re-create the visible differences from the command line mapserv.exe as you suggest . . . thanks again for the interest tim Tim Norris At 02:47 PM 10/17/2005, Frank Warmerdam wrote: >On 10/17/05, Kralidis,Tom [Burlington] <[EMAIL PROTECTED]> wrote: > > Thanks for the info. I don't think these instances are using > > WKT/GML as input or output, if that's what you mean. Basic .ovf > > connection to provide WMS output. Is there anything we can do on > > our end to help with this issue? The problem is basically: > >Tom / Tim, > >If you can reproduce the problem at the commandline with mapserv.exe >from FWTools then just bundle up the minimum data required to reproduce >it along with the commandlne required to invoke it and submit that as a >mapserver bug, reassigned to me. > >If you can't reproduce it under those circumstances then perhaps we can >work out a clue as to what is going from that. > >Best regards, >-- >---------------------------------------+------------------------------- >---------------------------------------+------- >I set the clouds in motion - turn up | Frank Warmerdam, [EMAIL PROTECTED] >light and sound - activate the windows | http://pobox.com/~warmerdam >and watch the world go round - Rush | Geospatial Programmer for Rent
