Digitalizar Datos del Mapa (QGIS3)

Digitizing is one of the most common tasks a GIS Specialist has to do. Often a large amount of GIS time is spent digitizing raster data to create vector layers that you use in your analysis. QGIS has powerful on-screen digitizing and editing capabilities that we will explore in this tutorial.

Vista general de la tarea

Usaremos un mapa topográfico ráster y crearemos varias capas ráster que representan entidades alrededor de un parque.

Other skills you will learn.

• Construir pirámides para grandes conjuntos de datos ráster para acelerar las operaciones de acercamiento y desplazamiento.

• Create and edit GeoPackage.

Consigue la información

Land Information New Zealand (LINZ) provee mapas topográficos ráster a escala 1:50.000 para la masa de tierra principal de Nueva Zelanda y las Islas Chatham.

Descargue el Archivo de Imagen geoTIFF de la página de descarga del mapa Christchurch Topo50.

Para su comodidad, puede descargar directamente una copia del conjunto de datos del siguiente enlace:

BX24_GeoTifv1-02-clip.tif

Fuente de Datos [LINZ]

Procedimiento

1. In QGIS, let’s load the image file. Go to Layer ‣ Add Layer ‣ Add Raster Layer.

2. In the Data Source Manager dialog select Raster. Under Source click on the ... and locate the downloaded BX24_GeoTifv1-02.tif and click Open. Then click Add followed by Close.

3. This is a large raster file, and you may notice that when you zoom or pan around the map, the map takes a little time to render the image. QGIS offers a simple solution to make rasters load much faster by using Image Pyramids. QGIS creates pre-rendered tiles at different resolutions, and these are presented to you instead of the full raster. This makes map navigation snappy and responsive. Right-click the BX24_GeoTifv1-02 layer and choose Properties.

4. In the Layer Properties dialog, Choose the Pyramids tab. Hold the Ctrl key and select all the resolutions offered in the Resolutions panel. Leave other options to defaults and click Build pyramids.

5. Once the process finishes, the dialog box will show the pyramids without a cross. This indicates the Image Pyramids build is completed. Click OK.

6. Antes de comenzar, necesitamos establecer las Opciones de Digitalización predeterminadas. Vaya a Configuración ‣ Opciones….

7. Select the Digitizing tab in the Options dialog. Check the Enable snapping by default under Snapping section. In Default snap mode choose Vertex. This will allow you to snap to the nearest vertex. I also prefer to set the Default snapping tolerance and Search radius for vertex edits in pixels instead of map units. This will ensure that the snapping distance remains constant regardless of zoom level. Depending on your computer screen resolution, you may choose an appropriate value. Click OK.

8. Now we are ready to start digitizing. We will first create a road layer and digitize the roads around the park area. Click Layer ‣ Create Layer ‣ New GeoPackage Layer… icon from Panels. A GeoPackage is an open, non-proprietary, platform-independent, and standards-based data format for a geographic information system implemented as an SQLite database container. This makes it much easier to move it around instead of a bunch of shapefiles. In this tutorial, we create a couple of polygon layers and a line layer so that a GeoPackage will be better suited. You can always load a GeoPackage and export layers as a shapefile or any other format you want.

9. In the New GeoPackage Layer dialog, click the … button and save a new GeoPackage database named digitizing.gpkg. Choose the Table name as Roads and select LineString as the Geometry type. The base topographic map is the EPSG:2193 - NZGD 2000 CRS.

10. When creating a GIS layer, you must decide on each feature’s attributes. Since this is a road layer, we additionally will have two primary attributes - Name and Class. In New Field Enter Name of the type Text data, with 50 as Maximum length and click Add to attribute list. Now create a new attribute Class of the type Text data, with 50 as Maximum length. Click OK

11. Una vez que la capa Roads esté cargada, clic el botón guilabel:Conmutar edición para poner a la capa en modo edición.

12. Clic el botón Añadir Objeto espacial Línea. Clic en el lienzo del mapa para agregar un nuevo vértice. Agregue nuevos vértices a lo largo del objeto espacial camino. Una vez que haya digitalizado un segmento de camino, clic-derecho para terminar el objeto espacial.

Nota

Puede usar la rueda del ratón para acercarse o alejarse cuando digitaliza. También puede mantener presionado el botón de deslizamiento y mover el ratón para desplazarse.

13. After you right-click to end the feature, you will get a pop-up dialog called Road - Feature Attributes. Here you can enter attributes of the newly created feature. Skip entering any value for fid as it is a sequential id that will be autogenerated. Enter the road name as it appears on the topo map. Optionally, assign a Road Class value as well. Click OK.

14. The default style of the new line layer is a thin line. Let’s change it to better see the digitized features on the canvas. Select the Roads layer and click Layer Styling Panel.

15. In the Layer Styling Panel, search for different road layer styles. Select topo road. Click OK.

16. Now the road layer will be clearly visible. If you are satisfied with the work, click Save Layer Edits button to save the changes.

17. Before we digitize the remaining roads, it is essential to update some other important snap settings to create an error-free layer. Right-click on any space on the toolbar area and activate the Snapping toolbar.

18. Now an Enable Snapping (Magnet Icon) will appear on the panel. Click on it to enable it and select All Layers and choose Open Snapping Options...

19. In the Snapping options dialog, click the Snapping on Intersection, which allows you to snap on an intersection of a background layer.

20. Now you can click Add feature button and digitize other roads around the park. Make sure to click Save Edits after adding a new feature to save your work. A helpful tool to help you digitize is the Vertex Tool. Click the Vertex Tool button and select Vertex Tool (Current Layer).

21. Once the node tool is activated, click on any feature to show the vertices. Click on any vertex to select it. The vertex will change the color once it is selected. Now you can click and drag your mouse to move the vertex. This is useful when you want to make adjustments after the feature is created. You can also delete a selected vertex by clicking the Delete key. (Option+Delete on a mac)

22. Una vez que haya terminado de digitalizar todos los caminos, clic el botón Conmutar Edición. Clic Guardar

23. Now we will create another layer to digitize the parks as polygons. Click Layer ‣ Create Layer ‣ New GeoPackage Layer… icon from Panels. In the New GeoPackage Layer dialog, click the … button and select the GeoPackage database named digitizing.gpkg. Name the new layer as an attribute called Parks. Select MultiPolygon as the Type. The base topographic map is the EPSG:2193 - NZGD 2000 CRS. Click OK. In New Field Enter Name, and the type as Text data, with 50 as Maximum length and click :guilabel:` Add to Fields List.`. Click OK.

Nota

Polygon vs Multi-Polygon

Polygon - Planar Surface defined by 1 exterior boundary and 0 or more interior boundaries. Each interior boundary defines a hole in the Polygon.

Multi-Polygon - It is used to represent areas with holes inside or consisting of multiple disjoint areas. For eg, 3 discontinuous polygons can be drawn and grouped as a single feature.

24. A pop-up dialog will appear. Select the Add New Layer button.

25. Now select layer Parks then clickroad Toggle Editing and click the Add feature button and click on the map canvas to add a polygon vertex. Digitize the polygon representing the park. Make sure you snap to the road’s vertices so there are no gaps between the park polygons and road lines. Right-click to finish the polygon.

26. Ingrese el nombre del parque en la ventana emergente Parks - Atributos de Objeto Espacial.

27. Now digitize the upper region of the park. Enter the park name and save the changes.

28. Now, before digitizing the inner polygon lets set settings that can ease this work. Multi-Polygon layers offer another useful setting called Avoid intersections of new polygons. Select Enable Snapping (Magnet Icon), click on it to enable it, and click All Layers and select Advanced Configuration.

29. Click the Avoid Overlap on Active layers button in the snapping toolbar.

30. Now in Edit Advanced Configuration, select the Units as pixels.

31. Check the box in the Avoid Overlap column in the row for the Parks layer.

32. Clic en Añadir objeto espacial para añadir un polígono. Con el Evitar Sobreposición, será capaz de digitalizar rápidamente un nuevo polígono sin preocuparse de ajustar exáctamente a los polígonos vecinos.

33. Right-click to finish the polygon and enter the attributes. Magically the new polygon is shrunk and snapped exactly to the boundary of the neighboring polygons! This is very useful when digitizing complex boundaries where you need not be precise and still have topologically correct polygon. Click Toggle Editing to finish editing the Parks layer.

34. Now it is time to digitize a buildings layer. Create a new polygon layer named Buildings by clicking Layer ‣ Create Layer ‣ New GeoPackage Layer… icon from Panels. Set the Buildings and MuiltiPolygon. Choose the CRS as EPSG:2193 - NZGD 2000. Click OK.

35. Once the Buildings layer is added, turn off the Parks and Roads layers to make the base topo map visible. Select the Buildings layer and click Toggle Editing.

36. Digitizing buildings can be a cumbersome task, and also, it is challenging to add vertices manually so that the edges are perpendicular and form a rectangle. We will use a QGIS toolbar called Shape Digitizing to help with this task. Right-click on any empty space on the toolbar area and activate the Shape Digitizing Toolbar.

37. Enable editing by pressing the pencil icon Toggle Editing.

38. Now under Add Rectangle dropdown select Add Rectangle from Extent button.

39. Zoom to an area with the buildings. Click and drag the mouse to draw a perfect rectangle. Similarly, add the remaining buildings.

40. You will notice that some buildings are not vertical, and we will need to draw a rectangle at an angle to match the building footprint. Under Add Rectangle dropdown select Add Rectangle from Center and a Point button.

41. Zoom to an area of diamond-shaped buildings. Click on the center to drop a point and drag the mouse to draw a rectangle.

42. Necesitamos rotar este rectángulo para hacer coincidir con la imagen en el mapa topo. La herramienta rotar está disponible en la barra de herramientas Digitalización avanzada. Clic-derecho en un área vacía en la sección barra de herramientas y habilite la barra de herramientas Edición avanzada.

43. Click the Rotate Feature button.

44. Use the Select Single feature tool to select the polygon you want to rotate. Once the Rotate Feature tool is activated, you will see crosshairs at the center of the polygon. Click exactly on that crosshairs and drag the mouse while holding the left-click button. A preview of the rotated feature will appear. Let go of the mouse button when the polygon aligns with the building footprint.

45. Save the layer edits and click Toggle Editing once you finish digitizing all buildings. You can drag the layers to change their order of appearance. The digitizing task is now complete. You can play with the styling and labeling options in layer properties to create a nice-looking map from the data you created.

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