Creating vector grid in QGIS for two different polygons without overlapping

Question

In QGIS I have the data for all the continents except Antarctica. Now I want to create a grid of size 1km * 1km for each country however the problem I am running into is that if I take two neighbouring countries the grids don't align properly as shown in the image below. Both grids and polygons follow the same CRS i.e. EPSG 4326 - WGS 84

I want to generate the grids in a way that it aligns with the other grids. I am complete beginner when it comes to GIS. I will get rid of the overlapping rectangles later, but I want these lines to align so that when shown on a map, it must look like one grid and not a collection of grids. Any guidance or tips?

Edit 1: I want to generate the whole data for world in the form of grids but I am unable to do so in one go as it always crashes after 2 - 3 days of continuous processing with only 6 -8 % progress. Any suggestions that can help me accomplish that will also be very helpful if this country by country route is too complicated.

Answer 1

Simply use PostGIS - run

CREATE TABLE global_grid AS (
  SELECT
    ROW_NUMBER() OVER() AS id,
    grid.i AS cell_x,
    grid.j AS cell_y,
    grid.geom
  FROM
    ST_SquareGrid(
      <CELL_SIZE>,
      ST_Transform(
        'SRID=4326;POLYGON((-180 -90, -180 90, 180 90, 180 -90, -180 -90))',
        <METRIC_UNIT_SRID>
      )
    ) AS grid
);

for a global grid, or

CREATE TABLE country_grid AS (
  SELECT
    cty.id,
    grid.i AS cell_x,
    grid.j AS cell_y,
    grid.geom
  FROM
    <countries_table> AS cty,
    LATERAL ST_SquareGrid(
      <CELL_SIZE>,
      ST_Transform(
        cty.geom,
        <METRIC_UNIT_SRID>
      )
    ) AS grid
);

to create aligning grids over <countries_table>.geom.

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Notes:

• Choosing a <METRIC_UNIT_SRID> is trickier than you may think, as there cannot exists an (equi)rectangular equal area grid covering a spheroidal body precisely - you may want to resolve using a Cylindrical Equal Area projection:

  • EPSG:54034, ESRI's default Spheroidal CEA projection
  • Behrends projection via proj4 string:
    '+proj=cea +lat_ts=30 +lon_0=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs'
  • Gall-Peters projection via proj4 string:
    '+proj=cea +lat_ts=44.138 +lon_0=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs'

• Do not create a grid! Especially when working with Point geometries that have to be referenced to the grid, getting the cell properties from rounding coordinates on-the-fly and lazily creating/storing cell geometries if needed is the much much better approach - especially especially if you need the grid reference mainly for metric aggregation and sporadic mapping. But also very generally!

Answer 2

Create one single grid worldwide (be aware: using EPSG:4326, you can't create grids in km, only in degrees. Reproject instead). In a second step, for each country (as you want a separate grid for each country, right?) make a selection based on location: select only grid cells overlapping with the current country.

To do so, you can use Menu Processing > Toolbox > Geometry by expression with this expression:

collect_geometries (overlay_intersects ('grid_layer', $geometry))

It creates one multipart geometry for each country, with overlapping cells where two countries border each other.

Grid (=cells in red) created for France, Spain and Portugal, selected (yellow) cells for France. Darker cells belong to two countries:

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Edit

Based on your additional information, be aware of this: As mentioned by @geozelot, creating 1km * 1km grid cells worldwide will simply create too large data to be handled. Either reduce the no. of cells (larger grid cells) or use a more performant computer system.

A workaround could be to first create a worldwide grid with much larger resolution (like 1000 km * 1000 km) and then subdivide it (100 km * 100 km etc.) for certain regions only until you reach the desired grid size. Based on the initial worldwide grid, cells will allign if you create the smaller grids based on the extent of one of these initial cells.

You will need to do this repeatedly for several regions of the earth (countries) separately. You could maybe create a model for ceratin steps to reduce reproducing repeating steps manually.