Ole:
A few more DEM generation tips... Try using two parallel rows of dense
points, fairly close together, to represent ridge lines. This has the
effect of "bounding" any overshoot if you are allowing it. Due to the
nature in which slope functions are calculated, it also allows the grid
surface to represent the peak more sharply. If you have lake boundaries of
known elevation then these should also be added to point file (using
poly2point) as you suggest. Include some estimated lake depths to control
the slope of the surface as it intersects the lake boundary. The lakes
should then be taken as a separate file and turned into a "sparse grid"
using Region2Grid (use the lake elevation column). This produces a grid
with only the lake surfaces, nicely flattened. Stamp the flat lake grid
into the previously generated DEM grid and you won't have odd lake/land
discontinuities where small "cliffs" of water poke out of the surface!
Accurate DEM generation should honour hydrographic features such as falling
rivers and not generate artifacts such as excessively large closed basins or
"sinks". The natural neighbour technique identifies closed depressions, so
maybe we can automate this some time in the future. We don't use it often
but do have an in-house routine to extract 3d point info from 3d DXF line
files of rivers. Someday we'll have time to clean-up and release this
utility (my product manager is going to shoot me for mentioning it). One of
the trickiest interpolation feats is gridding two-line rivers that fall
gradually along their watercourse. There are spatial routines for aiding
this but we haven't coded any yet.
Hope this is usefull,
Regards (from MapWorld),...
Phil Wyatt
Northwood Technologies Inc
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of Ole Gregor
Sent: April 29, 2000 2:38 PM
To: 'Simon W Fox'
Cc: '[EMAIL PROTECTED]'
Subject: RE: MI Vertical Mapper question
It is
Diitize your contour lines and add the heigts as an number attribute, add
lines along the coast and lakes and give them the corects heigth, add spot
heigths (tops and depressions), convert the layers to points with the
poly2point function in VM (and rember to get points at a certin distance
instead of just the nodes), do the interpolation (this is the difficult part
- I perfer Natural Neighborhood with no over/under shots for small areas
with a limited number of points, IDW with 4 zones, a few poits in each zone
and a limited search radius for a very large number of points, another
alternative is to do some aggregation and then use a TIN solution, but with
TIN there is a risk of spikes and over/undershots if some of the ponints is
close, has a big heigth difference and there is some way to the next points
(e.g. when you are going from a slope to af flat area).
The best way to learn about the different interpolation options is to read
in the manuals (the are reasonable good, but I don't agrre with their
recomendation of TIN to heigths if the data is from countor lines - there is
an old article in german about this specific problem and it's very easy to
make errors with any of the interpolation tools on the market (and
especially with AV Spatial Analyst and ArcInfo Grid where there is no easy
access to IDW with zoning), take some representive test data and perform the
interpolation with different methods, then do a point inspection on the
original points and look at the differences. It also a good idea to make a
copule of profiles and look after strange over/undershots. You will have to
take special notices to areas where the structure of the terrain is changing
(e.g. from steep to flat or where there is a lake/coast line or where there
is a dike/dam/..)
VM is a great very tool for interpolation and my favorite, but TIN with
breaklines would bee a nice new feature in the next version (IDRISI32 has
this option but is very, very slow compared to VM and can't deal with a
large number of points in the dataset).
Ole Gregor
Viborg Amt, Miljų og Teknik/Viborg County, dept. of enviroment,
natureconservation, planning and highways
(45) 87 27 13 07
-----Original Message-----
From: Simon W Fox [mailto:[EMAIL PROTECTED]]
Sent: Thursday, April 20, 2000 2:19 PM
To: '[EMAIL PROTECTED]'
Subject: MI Vertical Mapper question
Dear all
I have a question for any Vertical Mapper guru's out there.
Is it possible to create a grid/3D drape in vertical mapper from existing
contour lines. ie. digitised into Mapinfo from paper copy maps? If so,
could
you please give me some advice as to how to go about it.
Thanks for your responses
Simon
___________________________________
Rivers and Coastal Section
WS Atkins Consultants Ltd
Tel. 01925 622029
Fax. 01925 622054
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