topic Re: change polygon size? in ArcGIS API for Silverlight Questions

change polygon size?

SangamLama — Thu, 19 May 2011 14:55:14 GMT

Hi all,
how can I change the size of a polygon? I need to be able to increase/decrease it by user entered feet size. In case of a simple square/rectangle, I just add the entered value to XMax, XMin, YMax and YMin, but I'm not sure how I can do that with weird shaped polygons?

Thanks,

Re: change polygon size?

IgressT — Thu, 19 May 2011 15:24:53 GMT

Hi all,
how can I change the size of a polygon? I need to be able to increase/decrease it by user entered feet size. In case of a simple square/rectangle, I just add the entered value to XMax, XMin, YMax and YMin, but I'm not sure how I can do that with weird shaped polygons?

Thanks,

how about using buffering with some constraint on area (just a thought)

Re: change polygon size?

SangamLama — Thu, 19 May 2011 16:09:42 GMT

could you please elaborate? not sure I understand. You mean I first compute the area of the polygons, and then increase that?

how about using buffering with some constraint on area (just a thought)

Re: change polygon size?

IgressT — Thu, 19 May 2011 17:27:20 GMT

could you please elaborate? not sure I understand. You mean I first compute the area of the polygons, and then increase that?

Yes you can compute the area of the polygon using geometry service then increase the area

Re: change polygon size?

SangamLama — Thu, 19 May 2011 18:55:29 GMT

True, the area could be computed just as easily. I am more perplexed about how I can use the result to increase/decrease the polygon's size? Sorry I must be missing some simple point.

Thanks.

Yes you can compute the area of the polygon using geometry service then increase the area

Re: change polygon size?

IgressT — Fri, 20 May 2011 13:52:29 GMT

True, the area could be computed just as easily. I am more perplexed about how I can use the result to increase/decrease the polygon's size? Sorry I must be missing some simple point.

Thanks.

This is what I did and I don't know if its the right way or some one can suggest a better way but...
(Buffering creates round edges)

1. Draw some crazy polygon and calculate its area (a1)
2. Buffer it by say 1km and calculate its area (a2)
3. calculate the times increase in area x1=(a2/a1) which indicates for every 1km buffer your area increases by (a2/a1) times
4. ask the user how many sq km he wants to increase the polygon (a3)
5. calculate the times increase in area x2= (a1+a3)/a1
6. calculate buffer distance x2/x1
7. Draw the new polygon using the original polygon with new buffer distance

I wrote a small app for this and it works for me either if user wants to increase/decrease area

Re: change polygon size?

SangamLama — Fri, 20 May 2011 18:30:04 GMT

hmm. I'll certainly try that out. But before I do so, I should confess that I seem to have been looking at my issue from a stupid angle. What I really had been trying to do was draw a buffer polygon around any polygons.

In my case, these polygons are parcels. When user enters 'x' buffer size in feet and clicks on a parcel, it should essentially draw a buffer polygon around that parcel with the exact same shape, only bigger in area. A use case for this would be to determine what others parcels reside ten feet away from the selected parcel.

I was poking around with geometry server while calculating parcel's area, then I realized that it had a buffer function to meet my needs better. Using that, I tentatively achieved what I had needed, but it still isn't quite accurate.

BufferParameters bufferParams = new BufferParameters()
            {
                BufferSpatialReference = new SpatialReference(4326),
                OutSpatialReference = MyMap.SpatialReference,
                Unit = LinearUnit.Foot,

            };
bufferParams.Distances.Add(1); // Adding 1 foot buffer
bufferParams.Features.Add(parcelGeometry); // selected parcel's geometry
geometryService.BufferAsync(bufferParams);

this returns a geometry than can be drawn as a buffer polygon. It's very accurate if the buffer size is close to 0 feet. But bigger the buffer, the rounder the corners of the buffer polygon becomes. I have attached 2 screenshots here to show you what's happening -- left one with buffer size 10 feet, and the right with 50 feet.

In essence, I think Buffer function will give me what I need w/o a lot of trouble. So I was wondering if I could get some help preventing the corners from becoming rounder by feet?

This is what I did and I don't know if its the right way or some one can suggest a better way but...
(Buffering creates round edges)

1. Draw some crazy polygon and calculate its area (a1)
2. Buffer it by say 1km and calculate its area (a2)
3. calculate the times increase in area x1=(a2/a1) which indicates for every 1km buffer your area increases by (a2/a1) times
4. ask the user how many sq km he wants to increase the polygon (a3)
5. calculate the times increase in area x2= (a1+a3)/a1
6. calculate buffer distance x2/x1
7. Draw the new polygon using the original polygon with new buffer distance

I wrote a small app for this and it works for me either if user wants to increase/decrease area

Re: change polygon size?

IgressT — Fri, 20 May 2011 19:05:30 GMT

hmm. I'll certainly try that out. But before I do so, I should confess that I seem to have been looking at my issue from a stupid angle. What I really had been trying to do was draw a buffer polygon around any polygons.

In my case, these polygons are parcels. When user enters 'x' buffer size in feet and clicks on a parcel, it should essentially draw a buffer polygon around that parcel with the exact same shape, only bigger in area. A use case for this would be to determine what others parcels reside ten feet away from the selected parcel.

I was poking around with geometry server while calculating parcel's area, then I realized that it had a buffer function to meet my needs better. Using that, I tentatively achieved what I had needed, but it still isn't quite accurate.

BufferParameters bufferParams = new BufferParameters()
            {
                BufferSpatialReference = new SpatialReference(4326),
                OutSpatialReference = MyMap.SpatialReference,
                Unit = LinearUnit.Foot,

            };
bufferParams.Distances.Add(1); // Adding 1 foot buffer
bufferParams.Features.Add(parcelGeometry); // selected parcel's geometry
geometryService.BufferAsync(bufferParams);

this returns a geometry than can be drawn as a buffer polygon. It's very accurate if the buffer size is close to 0 feet. But bigger the buffer, the rounder the corners of the buffer polygon becomes. I have attached 2 screenshots here to show you what's happening -- left one with buffer size 10 feet, and the right with 50 feet.

In essence, I think Buffer function will give me what I need w/o a lot of trouble. So I was wondering if I could get some help preventing the corners from becoming rounder by feet?

Try using the Offset service with GeometryOffset.Mitered for "OffsetHow" option

Re: change polygon size?

SangamLama — Mon, 23 May 2011 13:40:27 GMT

Hi,

I tried Offset service instead but it didn't yield accurate results either. I've attached another screenshot of what I get after the new approach.

If you looked at the left polygon, the buffer's left line isn't quite parallel to the original polygon. And the right polygon's buffer goes haywire, I'm assuming because of the complicated shape.

Once again, the buffer polygon seems to be accurate if OffsetDistance is less, upto about 20 feet. Beyond that, the results aren't accurate.

Any way to make this better?

Try using the Offset service with GeometryOffset.Mitered for "OffsetHow" option

Re: change polygon size?

IgressT — Mon, 23 May 2011 13:49:20 GMT

Hi,

I tried Offset service instead but it didn't yield accurate results either. I've attached another screenshot of what I get after the new approach.

If you looked at the left polygon, the buffer's left line isn't quite parallel to the original polygon. And the right polygon's buffer goes haywire, I'm assuming because of the complicated shape.

Once again, the buffer polygon seems to be accurate if OffsetDistance is less, upto about 20 feet. Beyond that, the results aren't accurate.

Any way to make this better?

Don't know the reason for that... may be some one can answer it

Re: change polygon size?

SangamLama — Tue, 24 May 2011 12:49:20 GMT

yeah this is pretty bizzare. At least in case of the right polygon, I think what's happening is it's carrying out the projection by drawing numerous triangles and rectangles, but I gave the Simplify property of OffsetParameters as true, so that it would return final outcome only. That should have returned something simple.

I'd appreciate if someone had more insight on this as it's a bit urgent.

Thanks!

Don't know the reason for that... may be some one can answer it

Re: change polygon size?

IgressT — Tue, 24 May 2011 13:12:08 GMT

yeah this is pretty bizzare. At least in case of the right polygon, I think what's happening is it's carrying out the projection by drawing numerous triangles and rectangles, but I gave the Simplify property of OffsetParameters as true, so that it would return final outcome only. That should have returned something simple.

I'd appreciate if someone had more insight on this as it's a bit urgent.

Thanks!

Can you share the polygon data you are using to try it out

Re: change polygon size?

SangamLama — Tue, 24 May 2011 13:34:53 GMT

one thing now I'm curious about -- my map's measurements are in esriFeet unit. Is it the same as LinearUnit.Foot? I gave OffsetDistance value to be 250, but wasnt sure if those two units were the same..

Now, when you say polygon data, do you mean the following bit?

left buffer polygon:

{ESRI.ArcGIS.Client.Geometry.Polygon}
    [ESRI.ArcGIS.Client.Geometry.Polygon]: {ESRI.ArcGIS.Client.Geometry.Polygon}
    Extent: {1316977.40735295,281673.494292154,1317969.73234739,282601.656984727}
    SpatialReference: {ESRI.ArcGIS.Client.Geometry.SpatialReference}

{ESRI.ArcGIS.Client.Geometry.SpatialReference}
    WKID: 2285
    WKT: null

right buffer polygon:

{ESRI.ArcGIS.Client.Geometry.Polygon}
    [ESRI.ArcGIS.Client.Geometry.Polygon]: {ESRI.ArcGIS.Client.Geometry.Polygon}
    Extent: {1316566.86195633,278576.146840706,1317519.07831775,281886.679216881}
    SpatialReference: {ESRI.ArcGIS.Client.Geometry.SpatialReference}

{ESRI.ArcGIS.Client.Geometry.SpatialReference}
    WKID: 2285
    WKT: null

Can you share the polygon data you are using to try it out