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| Subject: | GISList: Creating a TIN in ArcView 3.x or 8.x with 3-d Analyst |
| Date: |
04/01/2003 07:50:00 AM |
| From: |
Morgan, John |
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Dear List Members:
Thanks to Bernard King, Margaret Gooding, Subash, Jayachandran Mani, Tadd
Berger, and Zheng Wu for their responses.
I received several helpful responses. To answer several of the questions
raised in the responses. The point files and contours were both from the
same LIDAR mapping. I was hoping to include the spot elevations due to the
very flat nature of the topography. The site is in southern Louisiana with
most of the ground below sea level and protected by levees. The levees are
the highest ground for most of the area.
I tried several tests on a portion of the area. When using the hard or soft
breaklines for the contour files, the file size and processing time was
similar. The appearance of the TIN did not seem to be any different. When
using the contours as mass points, the file is much smaller and the
processing is quicker. The TIN created in this way had several minor
inconsistencies in the results near the canals that are not acceptable.
What originally led to my question is that the point files were broken up
into about 60 shape files and the contours were in 7 shape files. ArcView
was not able to deal with that many files when creating the TIN. I would
get an error message that would stop the creation. I then tried creating
the outer boundary of the TIN with the point files and then doing multiple
runs to create the entire TIN. I was getting errors when trying to bring in
some of the other point files or contour files.
I finally figured out how to merge the shape files into one point file and
one contour file. I will be trying these consolidated files to see if this
solves some of my problems. The biggest problem I am having now is how slow
this TIN displays. The files are huge. The TIN is approaching 1 GB in
size. The area of interest is approximately 15 square miles. I have a
second I need to generate that will cover approximately 45 square miles.
In summary, it appears that I should treat the contour files as soft break
lines based on the responses and the minimal difference in appearance of the
TIN. I have enclosed the responses I received below. The original question
is at the end of the message.
**************************************************
I always use MASS POINTS without any problems.....but others may know much
more about the other options......
Bernard J. King
Senior Designer
CADD/GIS Dept.
MWH
Portland, OR
503.226.7377 ext.335
503.226.0023 fax
**************************************************
You will probably get the best results if you use mass points. Contours
would be okay, but will most likely be less accurate. Make sure you use a
reasonably low value for the interval (maybe the contour interval value).
Margaret Gooding
**************************************************
Hi John,
Hard breaklines are basically used to depict sharp changes in the
geographical terrain like cliffs from a valley or a shore from a rapid
flowing river. soft break lines are usually used to depict gradual changes
in the geographical terrain like undulating slopes etc.
So you have to keep this in mind when you decide on the type of breakline.
No this will not have any bearing on the size of the file substantially.
Regards,
Subash S
Manager - Technical Sales
Magnasoft
Bangalore, India
**********************************************************
hi,
it is better to use ur mass points for generation of
TIN. your contour, if created from the same set of
masspoints, creating a TIN from that is not of any
use. if ur contour source is something else other than
these masspoints and if it is more accurate, it is
better to use the contour for TIN.
But to create a perfect TIN (representation of
surface), both contour lines and mass points (which
may appear in between the conotur lines) can be used.
When you use, contours, ur TIN file size will be
larger comapred to creating the same from Masspoints
only
Regards
Jay
*********************************************************
John:
This is not my strongest subject area, but I will help answer just in case
an expert has not already responded.
If you are using contour lines, you want to treat them as soft break lines,
not hard. Hard break lines would represent ridge lines or valley bottoms,
where the slop clearly changes, while soft lines are points where the slope
passes through and may be near but not at the ridge top. Usually contour
lines on a topo are drawn as soft breaks.
As far as being better to use mass points or contour lines, that would
depend on the number of points or contours available. If the contour lines
were developed from the points theme, then just use the point theme. If
the point density is very close, use the point theme. If the points are
sporadic, but the contour theme is on a tight interval, say 1 foot, then
use the contour theme. Anywhere in between, which is what we usually get
in the real world, you will have to make a judgement call.
As f
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