Programmatically extract edge points from many polygons?

Question

Objective: Extract the four Latitude, Longitude points which correspond to the corner edges of each polygon in the Military Grid Reference System (MGRS) shapefile.

How can this be done in QGIS?

In this picture the red arrows point to the places I would like to know the Latitude, Longitude:

Here is what the MGRS shapefiles look like in QGIS 2.8.16, notice the corners look good:

When I used: y_min(bounds($geometry)) x_min(bounds($geometry)) y_max(bounds($geometry)) x_max(bounds($geometry))

And then plotted the data in Tableau Desktop, the result is some of the polygons are overlapping slightly and the corners aren't matching up exactly.

Here are the points which make up 14SPC:

GZD,LATITUDE,LONGITUDE,MGRS,PointID 14S,32.519328,-97.93512,14SPB,1 14S,33.434729,-99,14SNC,1 14S,33.420739,-97.924236,14SPC,1 14S,33.403103,-96.848967,14SQC,1 14S,34.322005,-97.912856,14SPD,1 14S,33.434729,-97.93512,14SPB,2 14S,34.341303,-99,14SNC,2 14S,34.336476,-97.924236,14SPC,2 14S,34.322005,-96.848967,14SQC,2 14S,35.238088,-97.912856,14SPD,2 14S,33.434729,-96.848967,14SPB,3 14S,34.341303,-97.912856,14SNC,3 14S,34.336476,-96.826229,14SPC,3 14S,34.322005,-96,14SQC,3 14S,35.238088,-96.802458,14SPD,3 14S,32.519328,-96.848967,14SPB,4 14S,33.434729,-97.912856,14SNC,4 14S,33.420739,-96.826229,14SPC,4 14S,33.403103,-96,14SQC,4 14S,34.322005,-96.802458,14SPD,4

Answer 1

You can use the field calculator to add this values to the attribute table of the layer (may be a virtual field is the best, because this values are recalculated when opening the attribute table).

Open the attribute table of your layer and select field calculator (the abacus icon):

Similar way you can create x_min, y_min and y_max. You can find the bound and x_max, y_max, x_min, y_min functions in the geometry group.

Answer 2

In QGIS, use the tool Extract Vertices to convert the vertices of your polygon layer to points. Make sure to save this layer in a geographic coordinate system.

You can view the coordinates of each point by clicking on it with the Identify tool.

To extract all the coordinates as a table, use the Field Calculator to add X and Y fields. Then save the vertices layer in CSV format. Now you have a CSV file that you can open with Excel.

Answer 3

In QGIS 3.x you can make use of side effects to extract edge points from polygons, when you copy&paste features from your polygon layer to a memory point layer:

layer = QgsVectorLayer("Point?crs=epsg:4326&field=name:string(255)", "nodes", "memory")
layer.addExpressionField( '$x', QgsField( 'x', QVariant.Double) )
layer.addExpressionField( '$y', QgsField( 'y', QVariant.Double) )
layer.addExpressionField( '$id - minimum( $id,group_by:="name") + 1', QgsField( 'id', QVariant.Int) )
QgsProject.instance().addMapLayers([layer])

The edge point Ids are autoicremented and start at 1 for every polygon. The big benefit is: you can always change the CRS of the memory layer before you copy the polygons.