projection woes

[Stephen Woodbridge]

Mark,

A couple of things you need to know, in case you don't already.

The PROJECTION block in the MAP section of the file is your output
projection. It does absolutely nothing unless you also have a projection
block defined in some of you LAYER blocks and it is different than the
output projection requested. Any LAYER that does NOT have a PROJECTION
block is assumed to be the same as the output PROJECTION.

Also, your EXTENTS need to be defined in the OUTPUT projection space NOT
the INPUT projection space. ALSO and CGI parameters need to be specified
in the OUTPUT projection space also.

So, if you do not have more than one projection space defined in your
mapfile, then NO projections are occurring.

Also from you description below, "NAD 27 zone 17" implies that you data
is in A) NAD 27 datum AND B) in a UTM zone 17 projection. Are you sure
your NAD 83 data layer is also in UTM Zone 17 projection OR is it
unprojected lat,long data? Typically, UTM data has coordinates like
1000000, 800000  and lat,long data has coordinates like +-90.0, +-180.0.

If you point disappears when you turn on a projection it typically means
that it skewed way off the your other data because you have specified a
projection that does not really match the data you applied it to. I
would go back and verify that your data is in the projection that you
are claiming it is in.

-Steve W.

Mark Adams wrote:
> Hoping someone can help me out with problems I’m having combining data from
> different projections. I have a set of layers in NAD 27 zone 17, which I’m
> displaying with a PROJECTION object set at the MAP level
> with "init=EPSG:26717". I’m trying to add a layer that is in NAD 83 zone
> 17. If I simply add this layer without specifying a PROJECTION, it displays
> fine, albeit out of sync with the underlying data due to the datum shift.
> If I add a PROJECTION object to the LAYER set with "init=EPSG:26917", the
> layer disappears – no error in the log file, just no data. The same thing
> happens if I add a similar PROJECTION object to any other layer – the data
> disappears.
>
> I’ve gone through the proj installation instructions, I can’t see anything
> wrong.
>
> Any suggestions?
>
> Thanks in advance.
>
> - Mark Adams
> markadams at cuestasys.com
>