Can DEM2TOPO use USGS data from Seamless to show topo data below sealevel?

With help from this group I have succeeded in making some topo maps for
my Garmin Etrex, but now I have a problem.

I will be going on a birding trip in the Salton Sea area of California,
and wanted to generate a map. The process works, but there are no
contour lines below an elevation of 0, which is where the interesting
areas are.

DEM2TOPO (v 2.9) has a setting for Sea level threshold, but setting it
to a negative value does not extend the contours.

Anyone have a suggestion for anything else to try, either in getting the
data from Seamless or in running DEM2TOPO?

TIA,

Gerry

On Wed, 16 Jan 2008 06:20:23 GMT, Gerry Snyder


Gerry

If you cannot find another way to do it, and you can export/import
either .shp or .dxf files, email me your points and I'll email you
back a file of countour lines above 0, below 0, or any combination.

- Tom

Hey,
I just had a go at the lake with data from NASA SRTM program. After some
contour calculations in Global Mapper, the contours down to 200 feet below
shows clearly. Maybe your input data is limited?
Regards
Thomas

On Wed, 16 Jan 2008 23:25:21 +0100, "Thomas"


Hey Thomas - what kind of weird-looking contours did you get? It
turns out the .HGT (90m spacing) files have all the correct negative
elevations also, so I don't know what kind of problem the OP might
have had.

When I contoured the .hgt data, I got lines wiggling around all over
the place, and zillions of little ovals where the land rose slightly
then dropped again, especially near the north & south ends of the
lake. I've never contoured anything so more or less FLAT before. It
would be nonsense to make an exact contour map out of this data -
you'd never know where you were, and would go blind looking at all the
lines. I did take the lake surface outline - at -71 meters on my data
- and filter the contour at that level, spline it, and pull/bend it
around to fit the shoreline quite accurately, so it results in a very
smooth but very accurate line. I was probably filtering out the
jitter in the original elevation sampling as much as in the actual
shoreline, hahaha.

I also set my contour generator to omit the drawing of any "small"
length contour lines - that took about 10 zillion of those little
ovals out that are probably mounds a few centimeters high right around
any given contour interval.

Overall I'd hve to suggest, that unless the OP had access to software
and the skill to do all this very professional and very time-consuming
work on the all the contours, and was happy with only a small portion
of the area where he will actually be bird-watching, I'd expect he'd
be spending far more time making maps than watching birds, hahahahaha!
(what's your hobby anyway!)

Wonder what Bonneville looks like on contouring - it's been graded
flat many times over the decades?

- Tom

On Wed, 16 Jan 2008 06:20:23 GMT, Gerry Snyder

Hey Gerry,

I just check GE, and neither Salton Sea nor Death Valley are showing
any elevations below 0, so I think you'll find it's your data, not
your program, in which case I can't help you.

- Tom