--- Begin Forwarded Message ---
Date: Wed, 02 Dec 1998 10:13:30 -0500
From: Tsering Wangyal Shawa <[log in to unmask]>
Subject: Re: Topographic Maps <fwd>
Mark may be right in pointing out problems with the Grid-> Contour
algorithms. Anyway, recently I downloaded and converted five or six 24k
DEM's in my area to gird. The conversion worked well with all of them
except one. In order to find out the quality of DEM data to 24k paper topo
maps. I converted DEM data -> Grid -> contour lines and compare those lines
with contour lines of paper topo maps. I found the result pretty good (I
am not sure about getting "exact" contour lines). My guess was, the problem
was not because of Grid-> contour algorithms but quality of data(I have to
do more research to prove this statement).
After converting DEM data into contour lines one could overlay TIGER95 or
97 data (although these data were originally digitized from USGS 100k,
still resolution and info are pretty good) of streets, rivers, block groups
etc. to get better idea of that area. As many of us know, information on
many of the hard copy 24k topographic maps were pretty old-- some of them
were not updated since 1970s or in some cases even since 1960s.
Most important things about working with digital data or using GIS tool is
-- We must ask this question to our patron. What level of accuracy a person
needs?
Thanks.
Tsering Wangyal Shawa
Geographic Information Systems Librarian
Digital Map and Geospatial Information Center
Geosciences and Map Library
Guyot Hall
Princeton University
Princeton, NJ 08544
Tel: (609) 258-3247
Fax: (609) 258-1274
http://www.princeton.edu/~geolib/gis
At 09:56 PM 12/1/98 -0500, Mark Eaton wrote:
>I wouldn't put much faith in converting USGS DEM's into the exact contours
that were used to generate the DEM's. We've been playing with the
Grid->Contour algorithms for some time and our success has been far from
outstanding. A lot of the problems have to do with the lack of
consistency in terrain and as we all know the earths surface is anything
but consistent. In regards to the availability of 24k DLG's, there is
roughly 7% coverage for the entire US. We have 100% coverage for 100k DLG
data for Hydro, Trans, PLSS & Bdys. If the Quads in question contain US
Forest Service Lands you can acquire CFF files from the FS as an
alternative. If your looking for 100% Topographic coverage that reflects
the consistancy and content of the 24k Topographic Map your only option for
quite some time will be the 24k Topographic Map. Sorry for the bad
news.....mark
>
>>>> Johnnie Sutherland <[log in to unmask]> 12/01 3:36 PM >>>
>--- Begin Forwarded Message ---
>Date: Tue, 01 Dec 1998 17:00:25 -0500
>From: Tsering Wangyal Shawa <[log in to unmask]>
>Subject: Re: Topographic Maps <fwd>
>
>
>What sort of topographic information person needs. If a person need
>elevation info s/he can download DEM data from usgs web
>page(http://edcwww.cr.usgs.gov/doc/edchome/ndcdb/ndcdb.html). Of course,
>all of the DEM 1:24,000 data are in sdts format. I hate this format(but
>there are free translator which you can download from above web site).
>Anyway, the sdts data can be converted into grid or ascii format then
>person could bring them in ArcView Spatial Analyst or Arc/Info GRID Module.
>You can either view them as DEM in a grid format or convert them into
>contour lines. You could create similar contour interval as topo sheet to
>get same contour line as USGS 1:24,000 Topo sheets.
>Besides, DEM data-- there are DLG data where you get info about boundary,
>hydrology, hypsography,
>transportation etc. All of the above data are free.
>
>Tsering Wangyal Shawa
>Geographic Information Systems Librarian
>Digital Map and Geospatial Information Center
>Geosciences and Map Library
>Guyot Hall
>Princeton University
>Princeton, NJ 08544
>Tel: (609) 258-3247
>Fax: (609) 258-1274
>http://www.princeton.edu/~geolib/gis
>
>>--- Begin Forwarded Message ---
>>Date: Mon, 30 Nov 1998 13:32:02 -0600 (CST)
>>From: Doralyn H Edwards <[log in to unmask]>
>>Subject: Topographic Maps
>>
>>
>>This may be a very naive question, but I'm not sure where else to go at
>>this point. I have a patron who is looking for Topographic maps of the
>>Crystal Crag, Bloody Mountain, and Mammoth Mountain regions in
>>California at the 1:24,000 scale in a digital format. From my searching
>>on the web, it doesn't appear that many topos are available on the web.
>>Does anyone know of a place I can get this on the web for free--otherwise,
>>we'll just scan them here (I'd rather not purchase them from a vendor,
>>unless reasonably priced).
>>
>>Thanks--I'm new at this and I'm not sure where else to go with the
>>question.
>>
>>Doralyn
>>
>>************************************************************
>>Doralyn H. Edwards phone: (713)737-5691
>>GIS/Data Librarian fax: (713)285-5902
>>Fondren Library email: [log in to unmask]
>>Rice University work: http://www.rice.edu/Fondren/GDC
>>Houston, TX personal: http://is.rice.edu/~doralyn
>>************************************************************
>>
>>
>>
>>
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>>
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