[GIS] Create an attribute table with coordinates vertices in QGIS

attribute-tableprint-composerqgis

I need to create, in a map layout, an Attribute Table item containing the Map Sheet vertices values in different coordinate reference systems, controlled by the atlas (Coverage Layer = Map sheet), as shown in the images.
Any advice to solve this problem?

I tried using vector-> geometry tools-> add geometry attributes but only area and perimeter values were added to the table, so I am considering using the vector-> geometry tools-> extract vertices first and then using the add geometry tool. I do not know if this is the right way to do this and I do not know how to get/add the converted coordinates (UTM and/or geographic) for all map sheets (the red rectangles) according to the other CRS.

Best Answer

Extract the vertices of your grid to a point layer with the extract vertices tool.

Remove duplicate points with the Delete duplicate geometries tool.

Use the Field Calculator and Expression Builder to add the desired coordinates to the point layer as attributes.

To get the X and Y coordinates in the same CRS as the layer, use the expressions: $x and $y.

To get the X coordinate of in ED50 / UTM 32 (EPSG:23032), use this expression: x(transform($geometry, 'EPSG:4326', 'EPSG:23032'))

I assumed the point layer is in WGS84. If not, change 'EPSG:4326' to the CRS of the layer.

Repeat for X and Y coordinates of all desired CRS's.

Add an attribute table for the point layer to your print layout. In item properties for the attribute table item, select "show only features intersecting atlas feature."

Format and arrange the attribute table to your specifications. To get it to exactly match the example, you may need to add multiple attribute tables, and incorporate other print layout items such as shapes and text boxes.

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QGIS – Drawing Rectangle with Specific Size from Attribute Table

@iant was faster but this is my version with PostGIS.

This one works with points and fixed offsets "1" to each direction.

select ST_GeomFromText('POLYGON (('
 ||ST_X(the_geom)-1||' '||ST_Y(the_geom)-1||','
 ||ST_X(the_geom)-1||' '||ST_Y(the_geom)+1||','
 ||ST_X(the_geom)+1||' '||ST_Y(the_geom)+1||','
 ||ST_X(the_geom)+1||' '||ST_Y(the_geom)-1||','
 ||ST_X(the_geom)-1||' '||ST_Y(the_geom)-1||'))')
from my_points;

This is using centroids and work with any geometry type:

select ST_GeomFromText('POLYGON (('
  ||ST_X(ST_Centroid(the_geom))-1||' '||ST_Y(ST_Centroid(the_geom))-1||','
  ||ST_X(ST_Centroid(the_geom))-1||' '||ST_Y(ST_Centroid(the_geom))+1||','
  ||ST_X(ST_Centroid(the_geom))+1||' '||ST_Y(ST_Centroid(the_geom))+1||','
  ||ST_X(ST_Centroid(the_geom))+1||' '||ST_Y(ST_Centroid(the_geom))-1||','
  ||ST_X(ST_Centroid(the_geom))-1||' '||ST_Y(ST_Centroid(the_geom))-1||'))')
from my_polygons;

If your offsets are stored into attributes "offset_x" and "offset_y" use this:

select ST_GeomFromText('POLYGON (('
  ||ST_X(ST_Centroid(the_geom))-offset_x||' '||ST_Y(ST_Centroid(the_geom))-offset_y||','
  ||ST_X(ST_Centroid(the_geom))-offset_x||' '||ST_Y(ST_Centroid(the_geom))+offset_y||','
  ||ST_X(ST_Centroid(the_geom))+offset_x||' '||ST_Y(ST_Centroid(the_geom))+offset_y||','
  ||ST_X(ST_Centroid(the_geom))+offset_x||' '||ST_Y(ST_Centroid(the_geom))-offset_y||','
  ||ST_X(ST_Centroid(the_geom))-offset_x1||' '||ST_Y(ST_Centroid(the_geom))-offset_y||'))')
from my_polygons;

[GIS] Extract the polygon layer vertices and delete the unwanted ones in QGIS

As your polygons are essentially square then running Processing -> Simplify on them using default tolerance (1.00) should be enough to remove the extraneous vertices and leave you only with corners (4 per polygon).

You can then use Processing -> Extract Vertices to create a point file of all vertices (albeit I don't know if this extracts the underlying polygon attributes). You can add geometry information using Processing -> Add Geometry Outputs.

EDIT

The original answer above works on small-scale cartographic coordinate systems such as British National Grid because generated grids are actually square/simple and therefore can be represented by only 4 vertices. In this case the data is in UTM 32N and closer inspection of the data reveals that the polygons are not regular due to the distortion from the projection. This appears to be the reason that standard methods such as simplification don't work. Because there is no standard pattern to vertex order, ramping up simplification tolerances ends up destroying a lot of the polygons rather than extracting corners.

The method outlined below does appear to work for the purposes laid out in the original question. It could be relatively easily automated as it isn't the most elegant:

Add a unique identifier to your polygon layer if they don't already have one
From Processing -> SAGA -> Vector General run the Feature Extents tool, using Each Shape as the creation option. This creates a new polygon layer which is essentially identical to the original but intervening nodes are trimmed out. Because of this there is a small degree of offset between the corners of each polygon (see image). Some spot testing suggests this is <50m.

If you add a new field to the attribute table of these new polygons and run num_points($geometry) you'll see that for some reason each polygon still has 5 vertices. Two of these per shape are identical, although where they occur and their index value is not standard. Testing indicates that Remove duplicate vertices doesn't seem to work regardless of tolerances; this may be a geometry error. Instead, to remove the duplicates run Processing -> Vector General -> Vector Geometry -> Extract Vertices to get the vertices of these new polygons.
Finally run Processing Processing -> Vector General -> Remove Duplicate Geometries to delete the duplicates.

You should be left with a point file where each group of four points represents the corners of a polygon grid square. There will be slight offset between the corners of each polygon but given the scale of your map you should be able to round this out when displaying the values (as per your original question linked in the comments). You will be able to identify each group of four using the unique ID you added in step 1.

Best Answer

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QGIS – Drawing Rectangle with Specific Size from Attribute Table

[GIS] Extract the polygon layer vertices and delete the unwanted ones in QGIS

EDIT

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