Common tasks for shapefiles and other vector formats.
This topic is being updatedMost of the recipes listed here can also be used for vector formats other than ESRI shapefile, including spatial databases. These formats can be opened with OgrLayer class while their data is stored in an instance of Shapefile class, which can be accessed using OgrLayer.GetBuffer property. See more information on opening different formats here.
The detailed API reference on classes, properties, methods used in the samples can be found here.
1. Opening of shapefiles and other vector formats.
For ESRI shapefiles:var sf = new Shapefile(); if (!sf.Open(@"d:\my_data.shp")) { Debug.Print("Failed to open: " + sf.get_ErrorMsg(sf.LastErrorCode)); return; }
For other vector formats:
var ogrLayer = new OgrLayer(); if (!ogrLayer .Open(@"d:\my_data.kml")) { Debug.Print("Failed to open: " + ogrLayer.get_ErrorMsg(ogrLayer .LastErrorCode)); return; } var sf = ogrLayer.GetBuffer(); // data is stored in instance an of Shapefile class
2. Adding layers to the map and retrieving them back:
For ESRI shapefiles:int layerHandle = axMap1.AddLayer(sf, true); // somewhere else in code to get this layervar sf = axMap1.get_Shapefile(layerHandle);
For other vector formats:
int layerHandle = axMap1.AddLayer(ogrLayer, true); // somewhere else in code to get this layervar ogrLayer2 = axMap1.get_OgrLayer(layerHandle); // or if you only need an underlying buffer with datavar sf2 = axMap1.get_Shapefile(layerHandle); // calls ogrLayer.GetBuffer under the hood
3. Changing visualization: colors, width of outline, etc.
Shapefile visualization options are stored in the instance of ShapeDrawingOptions class. It can be accessed via Shapefile.DefaultDrawingOptions property.var utils = new Utils(); sf.DefaultDrawingOptions.FillColor = utils.ColorByName(tkMapColor.Blue); sf.DefaultDrawingOptions.LineColor = utils.ColorByName(tkMapColor.Gray); sf.DefaultDrawingOptions.LineWidth = 2.0f;
In .NET it's not possible to assign System.Drawing.Color for these properties so we are using Utils class, other languages may set the colors directly.
3. Icons / markers for point layers.
TODO: describe4. Generation of labels.
TODO: describe5. Adding a visualization category.
To assign different colors to certain shapes within a shapefile you should use categories. By category we mean "visualization category", i.e. certain set of visualization options that are defined independent of any shapes. Categories can be managed using Shapefile.Categories property (returns instance of ShapefileCategories class). Each category is represented by instance of ShapefileCategory class and its drawing options can accessed with ShapefileCategory.DrawingOptions property.New category can be added with ShapefileCategories.Add method. Its drawing options will automatically be copied from Shapefile.DefaultDrawingOptions, i.e. if you set width to 2 previously, the new category will also have such width of outline.
// let's create a category with red fill color and green outlinestring categoryName = "red_shapes"; // any name can be used ShapefileCategory ct = sf.Categories.Add(categoryName); ct.DrawingOptions.FillColor = utils.ColorByName(tkMapColor.Red); ct.DrawingOptions.LineColor = utils.ColorByName(tkMapColor.Green);
6. Different ways to assign visualization category to shapes.
The category we created isn't assigned to any shapes, i.e. it's basically inactive. Here are various ways to change this:// let's assume that we want to assign our category to a 10-th shapeint shapeIndex = 10; // any of 3 overloads of Shapefile.ShapeCategoryint categoryIndex = sf.Categories.CategoryIndex[ct]; sf.set_ShapeCategory(shapeIndex, categoryIndex); // either (the fastest) sf.set_ShapeCategory2(shapeIndex, categoryName); // or sf.set_ShapeCategory3(shapeIndex, ct); // or// let's do the same in a cycle for shapes from 5th to 14thfor (int i = 5; i < 14; i++) { sf.set_ShapeCategory(i, categoryIndex); } // let's do the same based on attributes // the category will be assigned to shapes with Type field having value "hot"int fieldIndex = sf.get_FieldIndexByName("Type"); if (fieldIndex != -1) { for (int i = 0; i < sf.NumShapes; i++) { var value = sf.get_CellValue(fieldIndex, i).ToString(); if (value == "hot") sf.set_ShapeCategory(i, categoryIndex); } } // the same using expressions (under the hood this calls sf.set_ShapeCategory,// just like the previous one ct.Expression = "[Type] = \"hot\""; sf.Categories.ApplyExpression(categoryIndex);
7. Automatic generation of categories for a given field.
TODO: describe8. Adding markers to the map from latitude / longitude pairs of values.
TODO: describe9. Calculating area of polygons.
Depending on the presence of information about coordinate system /projection for the map the area for polygons can be calculated:- precisely using the shape of Earth;
- in projected coordinates using Euclidean geometry.
This examples demonstrates how to calculate area for polygons and write it to the attribute table of shapefile. If possible geodesic area is calculated.
if (sf.ShapefileType2D != ShpfileType.SHP_POLYGON) { MessageBox.Show("Area can be calculated for polygon shapefiles only."); return; } // DBF table must be in edit modeif ( !sf.StartEditingTable()) { MessageBox.Show("Failed to start editing mode for table."); return; } // this property can be used to determine whether transformation to WGS84// coordinate system is possiblebool ellipsoid = axMap1.Measuring.IsUsingEllipsoid; // create filed to store the resultsstring fieldName = ellipsoid ? "GeoArea" : "Area"; int fieldIndex = sf.EditAddField(fieldName, FieldType.DOUBLE_FIELD, 6, 18); // loop through shapes, calculate area and write it to the tablefor (int i = 0; i < sf.NumShapes; i++) { double area = ellipsoid ? axMap1.GeodesicArea(sf.Shape[i]) : sf.Shape[i].Area; sf.EditCellValue(fieldIndex, i, area); } // save the changes to the fileif (!sf.StopEditingTable()) { MessageBox.Show("Failed to save calculated area to the datasource."); }