Projection Help

[Giridhar Manepalli]

Thanks Ed!
That was one of the excellent articles I read anything about GIS. Now,
that I have known so much about Projection, I would like to explore
more on the same topic to learn more. If you have any pointers in
mind, kindly forward that to me. In the interim, I will try to grab
few online.

Once again, Thanks a lot.

Regards,
Giridhar


On Thu, 24 Mar 2005 22:30:45 -0500, Ed McNierney <ed at topozone.com> wrote:
> Giridhar -
>
> I think you are slightly misunderstanding the meaning of the EPSG codes.
>
> The term "map projection" is commonly used informally to describe a type
> or class of map projections.  For example, the Transverse Mercator
> projection is a method for projecting points on to a plane (a map), but
> there are many specific implementations of this class of projections.
> For example, a Transverse Mercator projection has a central meridian -
> you may select any line of longitude for that value.  Each choice will
> give you a different output image.
>
> In MapServer (and most other GIS usage) a "projection" refers to a
> specific implementation of a type of projection.  This basically
> describes the combination of a projection type (like Transverse
> Mercator) and a specific set of parameters (like the choice of central
> meridian) that go with it.  This meaning of "projection" refers to the
> set of ALL parameters required to completely, unambigiously describe the
> actual mathematical transformations used to create a map.
>
> These parameters can get wordy and easy to mistype or miscode.  It is
> also sometimes hard to recognize a familiar projection if it's only
> described parametrically.  The EPSG coding system addresses these
> problems.  Commonly-used projections are tabulated and each given an
> unique index number, the "EPSG code".
>
> For example the following definition:
>
> PROJECTION
> +proj=utm
> +zone=15
> +ellps=GRS80
> +datum=NAD83
> +units=m
> +no_defs
> END
>
> defines, in a MapServer map file, the UTM Zone 15N NAD83 projection.
> This is a common projection and has been assigned the EPSG code 26915.
> As a result, you could use this equivalent definition:
>
> PROJECTION
> +init=epsg:26915
> END
>
> The benefits are that the description is much shorter, it's easier to
> type correctly, and it's easy to see if two projections are really the
> same.  The disadvantages are that it's cryptic (and meaningless without
> an EPSG table) and it's impossible to use for "non-standard" projections
> that don't have a code assigned.
>
> So - now to get to your questions <g>:
>
> Q: Can anyone tell me how to set the EPSG or other information in the
> map file?
>
> A: By using a PROJECTION block as I have just shown above.  A PROJECTION
> block at the top level (not inside a LAYER) of the map file describes
> the projection of the output map image.  A PROJECTION block inside a
> LAYER block describes the projection of the source input data set.  If
> they are different, MapServer will do the reprojection for you (assuming
> it's properly configured, which is the normal case).
>
> Q: Can I know the Projection coordinates from the shapefile?
>
> A: The shapefile is really just a lot of coordinates.  I suspect you
> want to know if you can know the PROJECTION from the shapefile.  Often
> the answer is "No".  The shapefile format does not include projection
> information in any way.  Relatively recently, ESRI has started using an
> auxiliary file with the extension .PRJ (with the same base name as the
> shapefile) to describe the projection of the shapefile.  You will
> sometimes find these files with shapefiles - often you will not.
>
> Q: Is there a specific EPSG Code for each country or is it for each
> state in each country?
>
> A: There is a specific EPSG code for each projection.  You may be
> getting slightly confused by the adoption of standardized local
> projection systems.  Since you talk about Virginia, I'll give the United
> States example.  Each state in the US has adopted one or more State
> Plane Coordinate Systems (SPCS).  These are map projections that are
> used for surveying and legal coordinate documentation in each state.
> Each SPCS is designed to cover an entire small state or a portion of a
> larger state.  Map projections suitable for high-precision work are only
> suitable over a limited area - as you move farther from the natural
> origin of the projection, your errors increase.  In order to support
> highly accurate survey work, large states provide several "zones",
> usually all of which use the same class of projection with slightly
> different parameters.
>
> Q: How do I find the EPSG Code for Virginia or a city in Virginia?
>
> A: Therefore, you should know by now that Virginia does not have an EPSG
> code.  The State of Virginia has a State Plane Coordinate system divided
> into two zones - North and South.  To find out the boundaries of these
> zones, you can refer to an ESRI shapefile available at
> http://www.geographynetwork.com/data/download/esri/stateplane.zip
> showing the extent of all SPCS zones.
>
>         - 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
> ed at topozone.com
>
> -----Original Message-----
> From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
> Behalf Of Giridhar Manepalli
> Sent: Thursday, March 24, 2005 9:22 PM
> To: MAPSERVER-USERS at LISTS.UMN.EDU
> Subject: [UMN_MAPSERVER-USERS] Projection Help
>
> Hello All,
>
> I really appreciate the support that I have been getting in this list,
> without which it is impossible for me to go ahead in my project.
> Although, I have bulleted the questions that I have in my mind, any help
> in the discipline is invaluable to me.
>
> These are my questions:-
>
> Can anyone tell me how to set the EPSG or other information in the map
> file?
>
> Can I know the Projection coordinates from the shapefile?
>
> Is there a specific EPSG Code for each country or is it for each state
> in each country?
>
> How do I find the EPSG Code for Virginia or a city in Virginia?
>
> I appreciate if someone can throw some light onto this.
>
> Regards,
> Giridhar
>