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GIS Tech5 post(s)
#31-May-17 07:48

Hi there!

I would like to ask if is it possible to get/calculate the area of a Surface like DEM files. I recently found out that Manifold System 8.0 does not account the topography (Mountainous, Hilly, or Flat) of an area in presenting the AREA (I) value. So, my assumption is that Area (I) is not the same as Surface Area per se?

adamw


10,447 post(s)
#31-May-17 09:17

Area (I) is a flat area.

I would compute the surface area from slope (surface area of a pixel = flat area of a pixel / cos(slope), then sum values from all pixels under the desired object).

volker

1,086 post(s)
#31-May-17 20:03

out of my archive somewhere here in the forum (i dont't who the author was):

code sql

SELECT [ID],

   ( Avg( 1 / ( cos( Deg2Rad(

                      SlopeHeight([Dgm],[Dgm].[X (I)],[Dgm].[Y (I)], 1 )

   ) ) ) ) * First([Area (I)]) ) As [SURFACE_AREA] 

INTO [TRUE_AREA_M2]

FROM [POLYGON], [Dgm]

WHERE Contains( [POLYGON].[ID],

   AssignCoordSys(

      Geom( NewPoint( [Dgm].[Center X (I)], [Dgm].[Center Y (I)] ) ),

      CoordSys("Dgm" AS COMPONENT) ) )

GROUP BY [POLYGON].[ID];


http://www.thegisservicesector.de

volker

1,086 post(s)
#31-May-17 20:07

Have a look at the attached map.

Here the result of my drawing with the result of the query:

Attachments:
TRUE Area on Polygon with Surface.map


http://www.thegisservicesector.de

danb

2,064 post(s)
#31-May-17 21:06

This is something that I think would make an excellent Radian example or better still a built in function as I know with M8 it could become pretty slow when calculating the true surface area for large areas using queries.

Personally I think would be a great addition to the Radian armory and would certainly benefit my work. By way of an example, when we undertake hydrological catchment management and rehabilitation works, the first thing we do is calculate area and length statistics for various factors using conventional GIS techniques and hence planar areas/lengths.

These data underpin the design of work programmes and cost estimates which has always always troubled me somewhat. My suspicion is that we may inadvertently be substantially underestimating required resources as much of the local terrain is very undulose and we are often calculating requirements for larger catchment areas ranging from ~10K ha to many hundred thousands of hectares. With the way financing these projects works, accurate resource allocation at the start is often critical to the overall success of a rehabilitation project which may be several years down the line.


Landsystems Ltd ... Know your land | www.landsystems.co.nz

mao90210
84 post(s)
#15-Jan-18 09:52

Aside from surface area in the example above,

How about how to get the volume of the river using the same surface.map above?

Is it possible adding a query or a script?

Thank you

adamw


10,447 post(s)
#15-Jan-18 11:52

Create a geometry object covering the river, then use the Volume function in a query (Raster Extensions) to compute the volume of the surface under it (you will likely want to subtract the surface from some water level first).

KlausDE

6,410 post(s)
#15-Jan-18 15:20

The irregular slanting surface of a river would make this a perfect example for the use of 3D vector objects. And substraction of height of 3D vector objects (or a more general function with input of the direction of substraction) is an even more interesting feature for many mass balances.

I have always used raster functions with loss of accuracy in Mfd8 as workaround. A Significant loss with a systematic error in many scenarios.


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