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| GeoCommunity Mailing List |
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| Subject: | GISList: Contour Interval |
| Date: |
07/31/2001 09:00:29 AM |
| From: |
Joseph Loon |
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Some comments on Contour Interval:
1. If you use the National Map Accuracy Standards (NMAS) as a guide, then =
"..not more than 10% of elevations tested shall be in error by more than =
one-half of the contour interval." So if your heights have an accuracy of =
+- 5 feet (and assume that if you tested any height it would have the same =
accuracy) then the contour interval should be about 10 feet.
2. Another formula used is: RMSH (Root mean Square Elevation Error) =3D =
0.34 x Contour Interval. Assume your RMSH is 5 feet, then the contour =
interval should be about 15 feet.
3. Clearly the position of a contour is also affected by the positional =
accuracy of the map, and the contour interval is related to the map scale.
So, software programs allow us to draw in any contour interval but a too =
small contour interval will send a message to the user that the map is =
more accurate than it actually is. That is, in your case if you draw a 2 =
foot contour interval then you would be telling your map user that the =
height accuracy of your map is about 1 foot, if you use the NMAS.
4. If you are using the ASPRS Accuracy Standards for large scale maps ( =
1:60 to 1:20,000) then "The vertical check survey should be designed to =
produce Root Mean Square Errors in elevation differences at check point =
locations no larger than 1/20 of the contour interval." So in your case, =
if you were to draw a large scale map according to these standards (and =
your vertical accuracy is 5 feet) then you need to have a contour interval =
of 100 feet.
---------
To sum up: Contour interval is related to map scale, planimetric accuracy, =
vertical height accuracy and what standards you are working with. A good =
contour map should have this information on it so that the user has some =
idea what can (and cannot) be done with the map. In addition, there are =
formulas which take into account the maximum slope of the terrain being =
mapped when choosing a contour interval. If the area you are mapping is =
very steep and complex then a spacing of 25 feet may not be adequate. On =
the other hand, if the area you are mapping is fairly flat, then a spacing =
of 25 feet may be too dense. The spacing of the height points is also =
related to what heights you can eventually extract from the contour map =
(that is, the relationship between sampling interval and wavelength/amplitu=
de of the terrain).
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
*****************************
Joseph C. Loon, Ph.D.
Blucher Endowed Chair in Surveying
Professor of Geographic Information Science
Editor: Surveying and Land Information Systems
Texas A&M University - Corpus Christi
CI 312
6300 Ocean Drive
Corpus Christi, TX 78412
Voice: (361) 825-5854
Fax: (361) 825-2795
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