[mapserver-commits] r8700 - trunk/docs/tutorial

[svn at osgeo.org]

Author: pnaciona
Date: 2009-03-09 11:17:00 -0400 (Mon, 09 Mar 2009)
New Revision: 8700

Added:
   trunk/docs/tutorial/example1-6.txt
Log:
Add tutorial example 1.6

Added: trunk/docs/tutorial/example1-6.txt
===================================================================
--- trunk/docs/tutorial/example1-6.txt	                        (rev 0)
+++ trunk/docs/tutorial/example1-6.txt	2009-03-09 15:17:00 UTC (rev 8700)
@@ -0,0 +1,111 @@
+.. _example1-6:
+
+===========
+Example 1.6
+===========
+Defining Projections and Extents
+--------------------------------
+
+.. image:: http://biometry.gis.umn.edu/cgi-bin/mapserv.exe?map=/ms4w/apps/tutorial/htdocs/example1-6.map&layer=states&layer=states_label&layer=modis&mode=map
+
+Whoa! What happened here?
+
+You've just experienced on-the-fly reprojection. When a PROJECTION object is 
+defined in the mapfile, MapServer passes the layer and projection info to the 
+Proj.4 library which does the reprojection.  For more information about the 
+Proj.4 library, please visit `<http://proj.maptools.org/>`_.
+
+This example attempts to shed some light on the projection support in MapServer.
+
+This is what the mapfile looks like: `Example1-6.map <http://biometry.gis.umn.edu/tutorial/example1-6.map>`_.
+
+The first thing you might have noticed with our mapfile is the original EXTENT 
+has been commented out and the new EXTENT values don't look anything like 
+latitude and longitude values. Also, we have added a PROJECTION object near the 
+top and within each of our layers.
+
+Let's have a look at the new object and parameter:
+
+::
+
+      EXTENT  201621.496941 -294488.285333 1425518.020722 498254.511514
+
+The extent we provide to MapServer needs to be in the same units as the output 
+projection.  Since Lambert Azimuthal Equal-Area's units are in meters, we have
+to give the new extent in meters.  We can calculate the new extent using 
+ArcView or some other GIS package, or you can use Proj.4's cs2cs utility.  The 
+command below is what can be used to reproject the original extent values:
+
+::
+
+  cs2cs +proj=latlong +datum=WGS84 +to +proj=laea +ellps=clrk66 +lat_0=45 +lon_0=-100
+
+After typing the command, enter the southwestern coordinate pair (the lower 
+left coordinates), separated by a space:
+
+::
+
+  -97.5 41.619778
+
+to which the "cs2cs" utility gives the values:
+
+::
+
+  208398.01       -372335.44 0.000
+
+I then type the northeastern coordinate pair (upper right corner coordinates), 
+again separated by space: 
+
+::
+
+  -82.122902 49.38562
+
+and the following values are returned:
+
+::
+
+  1285308.08      632638.93 0.000
+
+You will notice that "cs2cs" returns a set of three values.  You can ignore the 
+third value, 0.000, as it means to represent altitute (which we're not using).  
+Anyway, we can not present the EXTENT as:
+
+::
+
+            EXTENT 208398.01 -372335.44 1285308.08 632638.93
+
+But it doesn't match up the extent above, you say.  Well, that's what usually 
+happens when you reproject--the map doesn't necessarily get centered as you'd 
+expect.  You can fudge around it, taking a few thousand meters off to the left 
+and adding a few to the right.  Or, you can use a graphical GIS package to give 
+you the extent. `Here are the instructions 
+<http://biometry.gis.umn.edu/tutorial/arcview_extent.html>`_
+for getting EXTENT using ArcView.
+
+MapServer can take projection definitions in two ways. The first is shown in the 
+mapfile's output PROJECTION object (the first PROJECTION block). This is the 
+traditional way of passing parameters to the Proj.4 library. The other way is
+to use the EPSG codes. These codes are standard projection codes (or spatial 
+reference identifiers) as defined by the European Petroleum Survey Group
+(EPSG). In the case of our Lambert Azimuthal Equal-Area projection, it has a 
+code of "2163". If you comment out the four lines after PROJECTION and uncomment 
+the line "init=epsg:2163", this will provide the same information to Proj.4. If
+you want to learn more about EPSG codes, have a look at 
+"/usr/local/share/proj/epsg" or "C:/proj/nad/epsg" ("/ms4w/proj/nad" in MS4W).
+
+Also, check out the EPSG web site at `<http://www.epsg.org>`_.
+
+If you need to learn more about projections, please look through some of the links below:
+`<http://www.colorado.edu/geography/gcraft/notes/mapproj/mapproj_f.html>`_
+
+`<http://www.geography.hunter.cuny.edu/mp/>`_
+
+`<http://www.nationalatlas.gov/articles/mapping/a_projections.html>`_
+
+`<http://en.wikipedia.org/wiki/Map_projection`>_
+
+`<http://erg.usgs.gov/isb/pubs/MapProjections/projections.html>`_
+
+`<http://spatialreference.org/>`_
+
+You can find more by searching the web for "map projection".