Catchment area 4

[4830]

There seems to be various methods for calculating the catchment area. And each method gives an area which varies in around 10% to other method. Can anyone give me which method is more appropriate and why?

 

[cvg]

do you mean drainage area? what methods are you talking about? Unless your mapping is terrible, you should be able to measure an area to better than 10% accuracy with multiple methods.

 

[4830]

yeah, i can say it is a drainage area. I didnt mean to say that my area was erroneous. It is just that i have used Planimeter to calculate directly from Toposheet and I have used DEM in GIS. And this two resulting areas varies within 10%. So, i am confused which one is accurate or let me say appropriate to consider for the design purpose.

 

[sam74]

I would use the DEM in CAD but I'm not overly familiar with the capabilities of GIS. That planimeter stuff is crazy. How big is the area? The bigger it is the more chance for error with the planimeter because of the human factor. I always thought we got that technology from the aliens at Roswell, NM.

 

[letrab]

Your error should be less than 10%. Use a square on the map 1km x1km and check it with the planimeter and CAD, maybe the scale used is incorrect but comparison with a square should indicate which method has the problem

 

[Ryb01]

I would only use a deliniation in CAD or GIS as a first check, to get a sense of the topography and magnitude of the drainage area.

To confirm the drainage area I would go into the field to verify and or modify the area based on what I've seen in the field. I've seen many cases where DEM and base mapping data has not picked up roadway culverts, ditch geometry and drainage divides which have increased and/or reduced drainage areas by a substaintial amount. Before going into the field, identify key areas where you would want to spend some time.

Hope this helps.

 

[brandoncdg]

How big is your area? I have only seen planimeters used to get a rough idea on an area, or doing earthwork by hand. Sometimes as a real world comparison to a CAD drawing.

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading

http://www.civildevelopmentgroup.com

http://www.civildevelopmentgroup.com/blog

 

[fel3]

Letrab has the right idea (or at least one of the right ideas). The planimeter could be out of calibration and needs to be checked against a known area.

However, an accurate electronic map (confirmed as Ryb01 suggests) will be far better than using a planimeter.

Now-a-days, I only use a planimeter for measuring off paper maps that don't scan well. If they scan OK, I drop them into Autocad, scale them, and make closed plines to get areas. Well, actually, I have other people do this for me.

 

[francesca]

There seems to be a lot of blind faith in CAD. A CAD-delineated drainage area is only as good as the surface it is based upon. CAD-delimited drainage areas tend to be jagged, which is not a true reflection of the drainage basin, just the limits of TINs. Planimeters are easy to calibrate. You draw a 1"x1" square and off you go. Digital planimeters, used on hand-delimited drainage areas, are fine. There's so much error inherent in the SCS or Rational method anyway that a little leeway on your drainage basin won't be the cause of the calculation being "wrong."

 

[fel3]

My practice has been to manually develop the drainage area boundaries in cadd by interpreting the contours. I don't yet trust software to do this.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[cvg]

I agree with FEL3, GIS based on DEM data does not always yield an accurate drainage area. And I always have to adjust things to account for diversions, culvert locations etc.

 

[MiketheEngineer]

If it's off by 10% - use the larger value but recheck.

Within 10% on any design job is usually cloes enough unless you are designing F15 wings.

 

[letrab]

Hi Mike
I agree that if you could estimate actual conditions to within 10% of the actual values you are doing well. However, if you start a basic process like measuring area and has a variance of 10 % you are in trouble. If you let your errors add up the chances are that your estimated final condition will be several orders of magnitude away from an engineering estimate.