USGS maps and aerial photos for PA

[Derrick J Brashear]

This is specifically for Bob Dietrich, but as others may be interested and
it's at least theoretically on topic I'll share it with all of you.

Now that I typed it I see it's a big wordy, and I'm sorry about that. As
we discovered yesterday you still find interesting stuff, like a set of
piers outside Millersville (near Lancaster) from a line which is of course
long since gone. 

The Pennsylvania Spatial Data Access system is hosted at the Deasy
Geographics Lab at Penn State. Their home site is http://www.pasda.psu.edu 
and they have an archive with includes all 7.5' series quadrangles for PA,
both collared and collarless, and all of the aerial photo quadrangle
quadrants (a.k.a. USGS DOQs)

Quadrangle maps can be found in
ftp://penne.deasy.psu.edu/pub/pasda/drg24k
and versions of these with collars clipped in
ftp://penne.deasy.psu.edu/pub/pasda/drg24k-c

The files are zip files named for the quadrangle contained therein, e.g. 
lancaster_pa.zip. Unzipping the file will give you generally a .tif file
and a .tfw file. Some may also have a .met or .mgd file, which are
metadata (like the date of the quadrangle). The .tif file is a TIFF image
file with GeoTIFF tags added. The GeoTIFF tags are essentially
georeferencing information, sufficient such that a suitably equipped piece
of software can know the coordinates of any point in the image.

For Windows users (of which I am not one) I know of at least 2 free data
viewers which can handle this information (note that any TIFF viewer can
be used if you don't care about the georeferencing, and since TIFF is a
standard format your Windows system might come with a viewer. I don't
know)

http://mcmcweb.er.usgs.gov/viewers/dlg_view.html is the USGS's map viewer
software.

http://www.esri.com/software/arcexplorer/index.html is from ESRI, the
largest GIS software maker probably in the world. 

The latter package is probably more refined. 

The latter package can also be used to view all of the aerial photos. They
come in 3 formats. One is the aforementioned .tif/.tfw style. The other 2
are ESRI "proprietary" formats, called .bil and .bsq (band interleaved by
line and band sequential). .bil and .bsq files come with a .hdr file with
some header information in it.

The aerial photos can be found at
ftp://penne.deasy.psu.edu/pub/pasda/doq

The files are again zip files, this time named by a quadrant of a 7.5'
quadrangle, e.g. lancaster_pa_nw.zip. They will contain either a .tif/.tfw
pair or a .hdr file with a .bil or a .bsq file. 

The resolution of the 7.5' quadrangles is 1 pixel equal to about 2.1
meters squared. The resolution of the DOQs is 1 pixel equal to 1 meter
squared. 

I expect to be able to burn custom CDs of these in the future, as I'm am
about 3/4 of the way through having a complete set of DOQs downloaded, and
I already have virtually all of the mid-Atlantic region's 7.5' quadrangles
downloaded. With non-standard TIFF "deflate" compression a DOQ file is
about 30 megs, meaning about 14 can be fit on a CD, or the equivalent of 3
and a half 7.5' quadrangles worth. When I get to the point where I have
all the maps and can burn CDs useable under Windows I will mention it to
the list, and can hopefully make up CDs for those of you who have some pet
area you're after. Oh, and I'll have to provide a TIFF inflator for you.

Theoretically this means that all of West Penn coke region territory could
be covered on 2 cds. I intend to try it. Note though, that even with 1m x
1m resolution it's not obvious from the air where the main line ran after
leaving 119 before it reached the street again in Dunbar (which is to 
say, the stretch where it crossed the PRR, B&O and WM in short order)

Must of the same data is available from the Microsoft Terraserver 
(http://terraserver.microsoft.com), but they don't yet cover all of PA
last I checked, their data is tiled into small JPGs, which means if you
want a large area you either have to capture an image or paste them
together, and at least one of the satellites they got source image data
from seems to have been a bit wacky when it passed near my house and
potentially elsewhere: a nearby corner doesn't mate nicely.

Note that all USGS data is freely redistributable, since your tax dollars
bought it.

You can send me any questions you have on the subject; As you might have
guessed this is an area of personal interest to me;-)

-D