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The area of feature don't equal the value Shape.Area ?

TaiBui

Hi everyone,

I used this statement to get the area value of polygon feature:

 while (pFeature != null)
     {
               
                IGeometry shapeCopy = pFeature.ShapeCopy;                                

                IArea area = pFeature.Shape as IArea;
                double value = area.Area; // Result is: 110102.807...
                  
       
                // ....
                pFeature = pFeatureCursor.NextFeature();
      }

But when showing the identify: The Shape.Area is: 20737.12 (not equal 110102.807). Please see the image:

[ATTACH=CONFIG]31505[/ATTACH]

Do you know what's wrong with it ?

Thanks and regards,

Tai

ShowIdentify.PNG

RyukiHIRAMATSU_0-1778807409967.png

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SachinKanaujia

Can you validate the following information?

1) Are you looking at the correct features (for matching)? Can you print the FIDs or does it have just one feature?

2) Are you changing the coordinate reference system in any way?

3) When looking at the values in ArcMap is there any conversion mapping or unit change occurring?

Please provide some more of this information to help understand the problem !!

TaiBui

Thanks Sachin,

Yes, it is the correct feature (polygon). But it contains many other features (polygons). So, I subtracted it the areas of sub features, but the final result doesn't equal the value: Shape.Area.

How can I get the area of that feature (not include area of sub features) ?

Please see my image (the current feature is green)

[ATTACH=CONFIG]31541[/ATTACH]

Thanks and regards,

Tai

Untitled.png

LeoDonahue

You need to set the spatial reference of your geometry object before calling the area method.

One thing you will notice about ArcObjects and the different data formats is that you get different area results for the same geometry, given the same projection.

Take that one geometry, and save it three ways: shapfile, file geodatabase and in SDE if you have it.

Then use ArcMap to get the Area of that geometry, you might see different results.

Now, use ArcObjects to get the area of that same geometry from the three different file formats and you will see slightly different results again.

SachinKanaujia

Will it be possible for you to export the data in a shapefile or the feature whose area you are calculating into a shapefile and upload it. Its difficult to understand how you are trying to accomplish it?

Also let me know what Spatial reference you are working in?

TaiBui

Thank you.

I am reading features from CAD file (.dgn) then write to feature class in SDE.

I haven't set the SpatialReference for the geometries of these features before checking the area.

Could you tell me how to set it ?

Plese see my codes:

Type factoryType = Type.GetTypeFromProgID(
                 "esriDataSourcesFile.CadWorkspaceFactory");
 System.Object obj = Activator.CreateInstance(factoryType);

factory = obj as IWorkspaceFactory;
IFeatureWorkspace ftrWorkSpace = (IFeatureWorkspace)factory.OpenFromFile(strWorkSpace, 0);
IFeatureDataset pFeatureDataset = ftrWorkSpace.OpenFeatureDataset(strFileName);
IFeatureClassContainer pFeatureClassContainer = (IFeatureClassContainer)pFeatureDataset;
IFeatureClass pCadPolygon;
IFeatureCursor pFeatureCursor = pCadPolygon.Search(pQueryFilter, false);
if (pFeatureCursor == null)
         return false;

IFeature pFeature = pFeatureCursor.NextFeature();
while (pFeature != null)
{
      ......
}

Thanks and regards,

Tai

LeoDonahue

If your data frame has a spatial reference defined, you can do something like this:

// Note: not actual code...
geometry.setspatialreference(the_active_view.getscreendisplay.getdisplaytransformation.getspatialreference)

SachinKanaujia

1) What is your source file (dgn file) spatial reference? Does it have an associated spatialreference file or not?

validate the source spatial reference

IGeometry shapeCopy = pFeature.ShapeCopy;

Get the spatial reference from Geometry to see if it has a geometry.

2) Your feature class in destination (feature class in SDE) must be in some spatial reference? What is that?

3) If the source and destination Spatial references are not same you will see differences. You will have to ensure that the CRS definition is correct. Depending of the spatial reference of source and destination you will have to either set spatial reference of project the shape to the target spatial reference.

TaiBui

Thank you,

The original dgn file haven't set coordinate system yet. Then I set it (same with projection system of SDE) as your say.

But, the area still not equal. So I am checking features again

I attached the project file for your refecence

Thanks and regards,

Tai

projectfile.txt.zip

isaacferrie

I am having this same type of problem. If I run the geometry calculator in the table I do not get the same area as when I run the
shape.area@acres in a script. They are close but they seem to be about .07 acres off. How do I run a script so that it gets me the exact same output as the calculate geometry feature?

Thanks

AlexanderGray

0.07 acres off sound like a rounding error. I don't think it is the same problem.
Shape.Area is usually almost meaningless. It is the cartesian area calculated in the coordinates of the dataset as if the the world was flat. If your dataset's spatial reference is not defined to start with, the shape.area is of unknown units, so potentially doubly meaningless. Just think, what does the shape.area mean if the polygons are in geographic coordinates (latitude/longitude)? It means pretty much nothing at all.
If your dataset is projected with non-angular units (metres, feet, etc.) then it represents the area within the tolerance of the distortion factor (scale factor) of the projection system if your projection system is appropriate for the data and area measurements. In that case you may end up with 0.07 discrepancies due to projection distortion.

isaacferrie

Thank you for the explanation. The projection is set for this project and view. I guess what I am wondering is there a script or command that would do exactly what the calculate geometry features does when working within the table.

Thank You

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