Filtering and Indexing GIS Vector Layers in C#
With Aspose.GIS for .NET you can filter layers by attribute values or spatial bounds. You can also use indices to speed up filtering and spatial queries.
Attribute index
Filter Without Index
Here’s how to filter a layer by values of an attribute:
// For complete examples and data files, please go to https://github.com/aspose-gis/Aspose.GIS-for-.NET | |
using (var layer = VectorLayer.Open(citiesPath, Drivers.GeoJson)) | |
{ | |
// Select features based on values of the attribute. This code enumerates all features in the layer | |
// and selects all features that match a condition. | |
var features = layer.WhereGreaterOrEqual("population", 2000).WhereSmaller("population", 5000); | |
// Print results. | |
Console.WriteLine("Cities where population >= 2000 and population < 5000:"); | |
foreach (var feature in features) | |
{ | |
var name = feature.GetValue<string>("name"); | |
var population = feature.GetValue<int>("population"); | |
Console.WriteLine(name + ", " + population); | |
} | |
Console.WriteLine(); | |
} |
Filter With Index
The code above is fine as long as layer is filtered only once. But, if layer is likely to be queried for multiple times, we can benefit from attribute indices. It takes some time to build attribute index, but it can be reused multiple times to speed up filtering.
The following example uses an attribute index file to speed up layer filtering by values of the attribute:
// For complete examples and data files, please go to https://github.com/aspose-gis/Aspose.GIS-for-.NET | |
using (var layer = VectorLayer.Open(citiesPath, Drivers.GeoJson)) | |
{ | |
// Use attribute index to speed up search by 'population' attribute. | |
// Aspose.GIS builds a new index if it doesn't exist, otherwise existing index is reused. | |
// Any path can be used. | |
var attributeIndexPath = Path.ChangeExtension(citiesPath, "population_out.ix"); | |
layer.UseAttributesIndex(attributeIndexPath, "population"); | |
// Select features based on values of the attribute. Since we use attribute index it is not necessary to | |
// test all features of the layer and filtering time is reduced significantly. | |
var towns = layer.WhereGreaterOrEqual("population", 2000).WhereSmaller("population", 5000); | |
// Print results. | |
Console.WriteLine("Cities where population >= 2000 and population < 5000:"); | |
foreach (var town in towns) | |
{ | |
var name = town.GetValue<string>("name"); | |
var population = town.GetValue<int>("population"); | |
Console.WriteLine(name + ", " + population); | |
} | |
Console.WriteLine(); | |
} |
Save Filtered Features
Filtered features can be saved into layers:
// For complete examples and data files, please go to https://github.com/aspose-gis/Aspose.GIS-for-.NET | |
using (var layer = VectorLayer.Open(citiesPath, Drivers.GeoJson)) | |
{ | |
// Use attribute index to speed up search by 'population' attribute. | |
var attributeIndexPath = Path.ChangeExtension(citiesPath, "population_out.ix"); | |
layer.UseAttributesIndex(attributeIndexPath, "population"); | |
// Save all features with population between 2000 and 5000 to the output file. | |
layer.WhereGreaterOrEqual("population", 2000) | |
.WhereSmaller("population", 5000) | |
.SaveTo(DataDir + "towns_out.shp", Drivers.Shapefile); | |
} |
Render Filtered Features
It is also possible to render filtered features. The following examples uses attribute index to quickly select all features with population greater than 2000 and add them to the map:
// For complete examples and data files, please go to https://github.com/aspose-gis/Aspose.GIS-for-.NET | |
using (var map = new Map(600, 400)) | |
using (var cities = VectorLayer.Open(citiesPath, Drivers.GeoJson)) | |
{ | |
map.Padding = 20; | |
// Use attribute index to speed up search by 'population' attribute. | |
var attributeIndexPath = Path.ChangeExtension(citiesPath, "population_out.ix"); | |
cities.UseAttributesIndex(attributeIndexPath, "population"); | |
// Render all cities with population greater than 2000. | |
map.Add(cities.WhereGreater("population", 2000), new SimpleMarker { FillColor = Color.Red }); | |
map.Render(outputPath, Renderers.Svg); | |
} |
Spatial Index
Spatial indices are used to speed up spatial queries. Just as attribute indices, spatial indices are reused after creation.
Find Features Nearest to Point
Here’s how to use spatial index to speed up looking for the feature closest to some point:
// For complete examples and data files, please go to https://github.com/aspose-gis/Aspose.GIS-for-.NET | |
using (var layer = VectorLayer.Open(path, Drivers.GeoJson)) | |
{ | |
// Use spatial index to speed up spatial queries. | |
// Aspose.GIS builds a new index if it doesn't exist, otherwise existing index is reused. | |
// Any path can be used. | |
var spatialIndexPath = Path.ChangeExtension(path, ".spatial_out.ix"); | |
layer.UseSpatialIndex(spatialIndexPath); | |
var point = new Point(12.30, 50.33); | |
// Since we use spatial index, nearest-to finds the closest feature much faster. | |
var nearest = layer.NearestTo(point); | |
Console.WriteLine("City nearest to (12.30 50.33) is " + nearest.GetValue<string>("name")); | |
Console.WriteLine(); | |
} |
Select Features Intersecting With Geometry
The following examples uses spatial index to speed up selection of features that intersect with geometry:
// For complete examples and data files, please go to https://github.com/aspose-gis/Aspose.GIS-for-.NET | |
using (var layer = VectorLayer.Open(path, Drivers.GeoJson)) | |
{ | |
// Use spatial index to speed up 'WhereIntersects'. | |
var spatialIndexPath = Path.ChangeExtension(path, ".spatial_out.ix"); | |
layer.UseSpatialIndex(spatialIndexPath); | |
var polygon = Geometry.FromText("Polygon((12.30 50.33, 22.49 54.87, 21.92 42.53, 12.30 50.33))"); | |
var intersecting = layer.WhereIntersects(polygon); | |
Console.WriteLine("Cities within " + polygon.AsText() + ":"); | |
foreach (var feature in intersecting) | |
{ | |
var name = feature.GetValue<string>("name"); | |
var location = (IPoint) feature.Geometry; | |
Console.WriteLine($"{name} at ({location.X}, {location.Y})"); | |
} | |
Console.WriteLine(); | |
} |