I'm still trying to track down the source of the mismatch between several of
my data sets, and in doing so have generated several new questions:
1. Does anyone know whether USGS DLGs for California (both the 1:24,000 and
1:100,000 scales) were originally created in NAD83 or NAD27? The original
paper quad sheets I've seen (1:24,000) are in NAD27, with a description in
the margins of the offset to the NAD83 grid.
2. Is anyone using the TopoDepot CDs? I've noticed that when I create a map
using the NAD27 datum and print it at 1:24,000 along with the 7.5 minute
quad boundary file from the USGS (which is in "unprojected" lat long), the
map elements line up very well with the paper versions of the quad sheets
(using the USGS boundary file to align the two maps on a light table).
However, the same TopoDepot map created in NAD83 shows the characteristic
NAD27-NAD83 offset (usually a few hundred feet) from the paper version. This
leads me to believe that the TopoDepot software simply changes the coordsys
settings, but leaves the actual coordinates the same, i.e. doesn't actually
reproject the data. Anyone else seeing this?
3. Along these same lines, I've received several GIS data sets (in NAD83)
from large public agencies in southern California, and these data sets line
up with TopoDepot's seemingly erroneous NAD83 maps (see point 2). At least
one of these GIS data sets was originally created in NAD27 and later
"converted" to NAD83, leading me to believe that these public agency GIS
data sets also not reprojected properly.
I'd love to hear from Cliff Mugnier, the MapInfo-l projection and datum
demigod, on this one.
TIA, and I promise to write a complete summary when I get to the bottom of
this.
_____________________________
Tim Warman
Geologist & GIS Specialist
Richard C. Slade & Associates
North Hollywood, CA
(818) 506-0418
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