Hi all,

This is for any of you GIS specialists out there!  Please help me if
possible, and feel free to email backchannel since this is somewhat
off-topic for both listservs.  I'm a bit of a newbie to doing GIS and
geomatics work.  As part of an effort to enable geographical browse,
search, and retrieval for some digital projects at LoC, I'm trying to
georeference a historical map [1] of the continental United States,
as well as parts of Canada and Mexico.

I plan to serve the map as a WMS layer, then pull it in using the
OpenLayers JS library inside our site for the end user navigation.  I
aim to "georectify" the image so that it can be placed over top of a
more modern map layer along the lines of Yahoo! Map Mixer [2] or
MetaCarta's Rectifier [3].  At this point I'll merely work with
raster data, since vector data isn't needed yet.

Here are the requirements this project:

-- I am required to use this historical map as the basis of my work,
as opposed to something more modern like Google Maps on its own.

-- I have to use open source tools, as I don't have ESRI tools at my
disposal.  Thus far my toolkit includes: Quantum GIS and uDig, GDAL,
PROJ.4, PostGIS, GRASS (although I'm not skilled at GRASS), and
Geoserver with PostGIS for serving the content.

Here's what I (think I) know, based on meeting with a GIS specialist
here at LC who has since departed for a new job:

-- The map has an Albers Equal Area projection, with NAD83 datum, and
GRS80 spheroid. (In proj or cs2cs: +proj=aea +lat_1=20 +lat_2=60
+lat_0=40 +lon_0=-96 +x_0=0 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m
+no_defs).  She determined this by opening the map in ArcGIS, and
it's ESRI Well Known Text (WKT) 102003.

-- The physical map was once folded, and might be throwing off some
of the measurements.

Using both gdal_translate (on the shell) and Quantum GIS (GUI)
georeference plugin, I've plotted upward of 40 ground control points
throughout the USA, Canada, Mexico, and Cuba.  I've found the WGS84
latlong coordinates on databases such as the GNIS as well as the
USGS.  US points are plotted to 5 decimal points of accuracy.
International points have 2 decimal points.  I run these through
cs2cs to convert latlong WGS84 to the Albers/NAD83 coordinates.
After warping the image based on these Albers/NAD83 ground control
points, I'll still end up as much as 50 miles off on my north/south
results, and around 3-4 miles east/west.

My question primarily has to do with best practices for
georeferencing a map that covers such a large area like North
America.    Have I used enough points?  Should I use more?  Or have I
already committed overkill?  Or, do I even have the right projection
information?  Would I be better off using some other tool(s)?

Thanks for any help you can provide.  I can provide more info, the
raster image, etc., if needed.

Clay

[1] http://hdl.loc.gov/loc.gmd/g3301p.ct001842
[2] http://maps.yahoo.com/mapmixer
[3] http://labs.metacarta.com/rectifier/