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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | Re: GISList: large datasets |
| Date: |
04/02/2002 03:16:39 PM |
| From: |
ðÅÔÒ ûÁÒÙÊ |
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Hugo,
Is there really a problem with SRTM grids? Initial (raw) SRTM images occupy 12
terabytes. There are 14,000 mosaics of the target drylands (1 mosaic is 1
degree by 1 degree terrain portion). Each mosaic at 90-m resolution is 2.75 MB
in 16-byte integers (not compressed): taken together, they will occupy 38,500
MB, that is, only one hard disk of usual PC for all the planet! You may store
this SRTM "large dataset" on several tens of CDs or on a couple of DVDs. In
32-bit real numbers, 2 times greater disk space is needed, but compression
essentially reduces this ratio (SRTM data are distributed as compressed files
.ZIP).
Each time you import an SRTM mosaic into some GIS (ARC/INFO, ArvView,...),
integer numbers are automatically transformed to real ones (e.g., .FLT file in
ArcView). So, in any case you store elevation grids as real numbers. Which
problem do you see with SRTM grids and with 90-m grids correcting to real
numbers?
I see real problems with rounded 90-m grids. In part, any attempt to use them
for hydrological purposes results in numerous artificial peaks in relatively
flat terrain parts, and there are no methods to objectively treat them (in
contrast to pits that can be filled).
Is there really a problem with large vector layers? (SRTM are only grids.) I
studied a special task on 30-m SRTM grid mosaic (108 km by 108 km), the task
was to create contour lines of a special land surface attribute (curvature).
ArcView 3.0a on 32MB PC fastly solved this task using a curvature grid from my
software: AV 3.0a has created 85,000 contour lines that contained about
14,000,000 nodes (the largest contour was about 105,000 nodes, 1000 km in
length), and returned them to my software as a shapefile.
Even 10-m grids are not a problem for USGS (90-m grids are 81 times smaller),
and you may download them free of charge from this USGS website
ftp://sdts.er.usgs.gov/pub/sdts/datasets/raster/dem/
that contains also free detailed vector layers for all the US (in non-compact
SDTS format, 19 CDs). Which problems do you see with such (or larger) vector
layers, or which your specific task encountered potential difficulties?
Peter Shary
Scientific researcher, and
GIS developer
Website http://members.fortunecity.com/eco4/giseco/
-----Original Message-----
From: Hugo Ahlenius <ahlenius@grida.no>
To: "'gislist@geocomm.com'" <gislist@geocomm.com>
Date: Tue, 2 Apr 2002 11:22:03 +0200
Subject: GISList: large datasets
>
> With reference to the thread about whole earth DEMs:
>
> Someone complained about the upcoming shuttle DEM would be integer only, and
> not floating point... I just thought for a second about the size of a
> dataset for the whole globe in 90m resolution with floating point...
>
> What are the records in the GIS business for the size of datasets? What is
> the biggest vector layer around (a theme in VMAP lvl 1?). Archives of
> imagery is of course massive, but apart from that?
>
>
>
> // Hugo Ahlenius
>
>
> -------------------------------------------------------------
> Hugo Ahlenius E-Mail: hugo@grida.no
> Project Officer Phone: +47 3703 5713
> UNEP GRID-Arendal Fax: +47 3703 5050
> Service box 706, N-4808 Arendal Mobile: +47 9964 7071
> Norway WWW: http://www.grida.no
> -------------------------------------------------------------
>
> ###########################################
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