Extract boundary from detailed layer in ArcGIS or QGIS

Question

Is it possible to return the boundary (outside) permiter of a polygon layer?

Let's say you have the map of the USA, for simplicity, broken down to the county level, is it possible to return the boundary map of the whole country from that map?

Answer 1

You can just do a dissolve on the county layer (be sure to have just the counties you want to dissolve selected).

Answer 2

Your getting lines when you do a dissolve because the boundaries are not seamless.

Regardless of software product, do the following:

Do an integrate.

Then do a dissolve.

Answer 3

Allow me to plug my own project... Boundary Generator will give you both external and internal boundaries of every polygon (as line features).

Internal boundaries are attributed with the FID for each of the two polygons that share that border. External boundaries should have an FID of zero for one of those two, so they are easy to select out of the full result.

The nice thing about it vs doing dissolves is that I've added a couple of knobs over precision so it can deal with not-quite-perfect data. (How far apart should two polygon borders be to be considered a shared border? How much angle deviation is required?)

It's still in alpha and it's been a while since I've tackled an update; I'd love to hear how well it works for you!

Answer 4

You can use GRASS module: v.dissolve You only need to import your data into GRASS. Here is a screen-cast of transferring your data to Grass from inside QGis.

Answer 5

 public static IPolygon getPolygonFromLayer(ILayer layer)
{
            IFeatureLayer FLayer = layer as IFeatureLayer;
            IFeatureClass FClass = FLayer.FeatureClass;
            return polygonMerge(FClass);
}

   private static IPolygon polygonMerge(IFeatureClass featureClass)
        {
            if (featureClass == null) return null;
            IGeoDataset geoDataset = featureClass as IGeoDataset;

            //You can use a spatial filter to create a subset of features to union together. 
            //To do that, uncomment the next line, and set the properties of the spatial filter here.
            //Also, change the first parameter in the IFeatureCursor.Seach method.
            //ISpatialFilter queryFilter = new SpatialFilterClass();

            IGeometry geometryBag = new GeometryBagClass();

            //Define the spatial reference of the bag before adding geometries to it.
            geometryBag.SpatialReference = geoDataset.SpatialReference;

            //Use a nonrecycling cursor so each returned geometry is a separate object. 
            IFeatureCursor featureCursor = featureClass.Search(null, false);

            IGeometryCollection geometryCollection = geometryBag as IGeometryCollection;
            IFeature currentFeature = featureCursor.NextFeature();

            while (currentFeature != null)
            {
                //Add a reference to this feature's geometry to the bag.
                //Since you don't specify the before or after geometry (missing),
                //the currentFeature.Shape IGeometry is added to the end of the geometryCollection.
                object missing = Type.Missing;
                geometryCollection.AddGeometry(currentFeature.Shape, ref missing, ref missing);
                currentFeature = featureCursor.NextFeature();
            }

            // Create the polygon that will be the union of the features returned from the search cursor.
            // The spatial reference of this feature does not need to be set ahead of time. The 
            // ConstructUnion method defines the constructed polygon's spatial reference to be the 
            // same as the input geometry bag.
            ITopologicalOperator unionedPolygon = new PolygonClass();
            unionedPolygon.ConstructUnion(geometryBag as IEnumGeometry);

            return unionedPolygon as IPolygon;
        }

    }

Answer 6

It sounds like the Bounding Containers sample posted on arcgis.com will work for what you want.