projection questions

[Ed McNierney]

Jeff -

 

Unfortunately, ESRI forgot about projections when the shapefile format
was created.  There was a later addition of a .prj file to go along with
shapefiles and describe their projection.  If you have a .prj file of
the same base name, you will have that information - I expect you do
not.

 

Without that information you're down to educated guesses.  There are
three likely candidates for US data - geographic (unprojected)
coordinates, a UTM projection, or a state plane projection.  If you are
dealing with a multiple-state area, state plane is unlikely.

 

Can you examine the coordinates in the shapefile by either using the
shpdump utility or loading the shapefile into a GIS viewer and reading
the coordinates?  For New England, geographic coordinates will have X
values ranging from around -74 to -66, and Y values between  41 and 48.
UTM coordinates will range between X values of 200000 to 700000 and Y
values between 4500000 and 5250000.  As you can see, these are obviously
different and easy to distinguish.

 

If you find you have values that don't match either of these, let us
know what they are; the shpdump utility will give you the bounding box
of all shapes in the shapefile, and that's a good guide.

 

-          Ed

 

Ed McNierney

TopoZone.com

 

________________________________

From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Jeff Portwine
Sent: Friday, January 21, 2005 10:49 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: [UMN_MAPSERVER-USERS] projection questions

 

 

Is there an easy way to figure out what projection my shapefile was
created with?   I have a map of the New England area that was created in
Microsoft's Map Point, and I am more or less trying to re-create that
map with mapserver.   I started with an outline of the United States and
then set the extents to display the area that I wanted, but the maps
look very different and i'm positive it's becuase I'm not using proper
projections  (I don't expect the maps to look exactly alike of course,
but the general shape should be pretty close).    I always use ogrinfo
to get the extents and see projection information of shapefiles that
I've downloaded but more often than not , the ogrinfo just returns:
"Layer SRS WKT:  (Unknown)"   instead of telling me the projection it
was created with.

 

I also am not sure how to go about choosing my output projection...
I've been using EPSG:4269 because as I understand it that is a midwest
projection and when I first started playing with mapserver I was making
maps of Michigan.    Anybody know a good source to look up what would be
the best output projection for different areas of the country/world?  

 

As you can tell i'm really new to GIS and while I do understand the idea
of projections I just don't know anything about them or how to apply
them.

 

Thanks a lot,

Jeff

 

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