The process of digitizing polygons is similar to that of polylines. In the Create Features panel, select the layer (e. g. plant_cover) and choose Polygon in the Construction Tools. To start, mark the first vertex with a click and then continue drawing the entire perimeter of the area. To finish, double-click or press the “F2” key. You can define several categories (e. g. Figure 4: Grassland, crops and forest).
In the case of polygons sharing one side, it is necessary to ensure that there are no overlapping surfaces or empty spaces in the boundaries of the polygons.
Gaps or empty spaces are not usually visible to the naked eye, but only when there is a close approximation to the boundaries of the plots. Similarly, areas shared by two or more polygons may occur, which is called overlap (see Figure 20). These topological errors usually occur when no methods for maintaining the topological rules were used in the edition. The same may occur when compiling information from different sources or scales.
There are several tools in ArcMap that allow you to maintain integrity and quality when editing spatial information. Two fundamental tools are located in the Editor bar (Cut Polygons Tool and Reshape Feature Tool) and another one in the section Construction Tools (Auto Complete Polygons) that allow a polygonal layer to be successfully digitized, significantly reducing the amount of errors that are usually generated in this activity. When attempting to draw a surface with subdivisions, it is preferable to first create the external perimeter and then make the internal division. For example, when building a land use map, it is recommended to draw the entire study area and then divide into categories. To carry out this process, activate the Edit Tool() in the Editor bar and select the required polygon (the edges of the polygon turn light blue). Then click on Cut Polygons Tool to proceed. To cut the polygon, mark the first point outside the polygon and continue drawing a cutting line inside the area. The last point must be outside the polygon (see Figure 21). The cut is completed with a double click or by pressing the “F2” key. The process can be repeated as many times as necessary until all required divisions of the area are made. It should be emphasized that this tool is only activated when one or more polygons are selected.
Editing polygons also requires knowing how to change their shape, especially when certain geometric readjustments are required. To do this, select the polygon (with the Edit Tool) and then click on the Reshape Features Tool icon () located on the Editor toolbar. An example of how to change the shape of the polygon is given in Figure 22. In the first two polygons on the left, it is shown how to increase the area of a polygon (in this example: a square). The important thing is always to start and finish the readjustment inside the polygon. On the right-hand side, the last two polygons show how to reduce the area of a polygon (in this example, to obtain a square shape). For the readjustment, you must start and finish outside the polygon.
If polygons need to be created next to each other, do not draw a border twice, but maintain a continuous structure (no overlapping or gaps). To create continuous polygons, go to the Create Features panel, in the Construction Tools section check the Auto Complete Polygons () option. To start drawing the new continuous polygons, it is not necessary to select the entities. The new polygon is drawn with at least two vertices connecting it to its neighbors (Figure 23).
Additionally, it is often necessary to merge two or more polygons into a shapefile. To do this, select the participating polygons by pressing the “Shift” key during selection. The Merge tool is then applied from the Editor toolbar (Editor > Merge).Practice: In the plant_cover and lake layers, digitize the polygons, making use of the editing tools. When you have finished all the activities involved in digitizing points, lines or polygons, finish editing with Stop Editing located in the Editor toolbar (Editor > Stop Editing).
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