projection confusion...

[Mark Dieterich]

Evening everyone,

I'm hoping some of you can help dig me out of my utter confusion ;) 
I've been humming along, working with mapserver for a while and never 
really giving projections much thought.  Then came tiling...  It 
requires a constant mapping between pixel values and the geographic 
space.  Enter confusion...

I'm working with tiger data, national atlas data, and some home grown 
data (based off the tiger data, so it will have the same projection as 
the tiger data).  So far, all I've managed to do is draw blank maps, so 
I know I either have projection mis-matches or extents which are foobar.

The first issue I have is, how do you determine what projection the data 
is already in?  I've seen about three different answers to the question 
re: national atlas data; none of them seem to make any difference for 
me.  I've tried playing with ogrinfo to query the underlying shapefiles 
and haven't found anything remotely helpful.

I decided to follow the all mighty google and have my output projection 
be in a mercator projection, so I have this in my mapfile:

   PROJECTION
     "proj=merc"
     "lat_ts=0.0"
     "lon_0=-90.0"
     "x_0=0.0"
     "y_0=0.0"
     "ellps=GRS80"
     "no_defs"
   END

which I think is correct.  Now I figure I just need to get my various 
data layers defined with an appropriate projection and life might get 
better.

As for my second issue, how do I convert from decimal/degree extents to 
meters?  I tried running something like this:

proj +proj=merc +lat_ts=0.0 +lon_0=-90.0 +x_0=0.0 +y_0=0.0 +ellps=GRS80 
+no_defs

and then entering in my lat/long values.  It returned some really nice, 
big, numbers, but I haven't a clue whether they were right.  Again, 
blank map.

I clearly have a lot to learn about map projections, but I've spent 
many, many, many hours searching around the net trying to answer these 
questions and it's only led to more frustration.  Can anyone out there 
offer up some help?

Thanks!

Mark