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DEM vs TIN from contour

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#1
Guest_dowlinga_*

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Hi

Working on a 3d terrain map for some 3d visualisation for a personal development.

Have
painstakingly traced in 20m contour map and assigned elevation data.
Have tried a few programs to generate a dem of the area. I can end up
with a resonable representation, but the end result shows quite a lot
of terracing. I can export as a greyscale heightfield bitmap and do a
guassian blur on photoshop to improve this, however dont want to lose
accuracy.

So my questions as follows.
Which is the best
method to interpolate contour lines? Is anudem considered the best for
this? Do you think I will get a better result creating a TIN?

Will
then need to convert to a 3d mesh. havent decided what the final render
will be in think it will be in vue d'esprit or lightwave.

thanks for any help or tips..



#2
Martin Gamache

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When
you say Anudem, do you refer to the original program or the ESRI
spatial analyst implementation (topogrid). I've had good results with
the latter especially when I also use a hydrological dataset and spot
elevation points as inputs and do not insist on a hydrologically
correct DEM. There are some settings that will result in a smoother
dataset...There are also some very good algorithms in GRASS for doing
this and also Surfer that others on this board may have more experience
with...maybe they will chime in.

#3
Clark Geomatics

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"

Which 
is the best method to interpolate contour lines? Is anudem considered 
the best for this? Do you think I will get a better result creating a 
TIN?

http://

TINs
work well for terrain data and for data points that have a linear
progression or relationship to each other across the grid, such as
temperature. A TIN produces triangles from a network of points that
more closely reproduces the original map terrain than a IDW or bicubic
interpolator. It draws lines (straight) between points, dividing them
into triangles and connecting all the points that it can. It creates a
mesh of connectivity so that the grid points can be interpolated. The
interpolation is not influenced by the neighboring original data
values, so that false highs / lows aren't created.

However, you
are creating your DEM from existing contours and my guess is that you
will need to interpolate between points - a TIN will just connect
points and likely enhance flat areas. I've used the BlackArt product (a
bit of a learning curve when it works) with satisfactory results - it
will take care of the terracing you are experiencing.

And, as
Martin suggests, GRASS will produce good results from its interpolation
algorithms - again, a bit of a learning curve to get the software
installed / working. "
Cheers,

Jeff Clark
Principal
www.clarkgeomatics.ca

#4
franciscocartographer

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"I am no expert but this is my personal opinion based on experience.

If you have access to ArcInfo, TopoGrid will be the best choice, no question.
I
do not have access neither to ArcInfo or ANUDEM so I did use a TIN for
creating an elevation model based in 10 m contours (from DLG) some time
ago.

The key is to use the contours as ""softlines"", spot heights
(elevation points) as ""mass-points"", and Hydrography as ""hard lines"".

The more contours lines you have the better the results will be and you will end up with less flat areas and terracing.


There is a good discussion of this (if you can read spanish) in:

[url="http://"http://foro.gabrielortiz.com/topic.asp?TOPIC_ID=3513"]http://foro.gabrielortiz.com/topic.asp?TOPIC_ID=3513[/url]

Francisco.

"
--------------------------------------
Francisco Jimenez, GISP
Senior GIS Analyst & Amateur Cartographer

My webpage

#5
Guest_dowlinga_*

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Thanks for all your comments.

I
havent tried using the spot points, will try adding these in as well. I
havent got hydrography data, but I have got tracings of the streams
& river. This doesnt have elevation data but do you think I may be
able to use it?

Alpine Mapping Guild - was refering to anudem used in arcinfo. Would you be able to expand on the smoothing options in topogrid?

Sorry dont read espanola but will see how good translation is in babelfish...







#6
Martin Gamache

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"By
hydrography I meant vector data of streams and lakes...Using streams
and lakes can help define gulleys and flat areas in your DEM. It's not
necessary to have elevation values for these but it wont hurt
especially for the lakes. Just make sure you correctly identify the
type of input to the algorithm. Topo lines should be topolines, points
should be points etc... there are specific categories for rivers and
lakes.

Just to be clear there is both both the arcinfo topogrid
function and the one implemented in spatial analyst toolbox as ""topo to
raster"" (I think that is the name) which has a detailed help menu with
a good description of the various parameters. I'm not sure how
different these are, the toolbox version is much easier and faster to
use in my experience.

If you are running this form the arcinfo
command line there are also some description but I recommend
experimenting with a small subset of your dataset as it can be slow to
calculate for a large area. "




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