How to emulate a specific vector grid in QGIS?

Question

I would like to analyze data included in a pdf file. The file contains information on characteristics of 16km grid cells for a specific study area. The file also includes a map displaying the grid. I know the coordinate system and datum that has been used to create this map.

I would like to create the exact same grid in QGIS as to merge the data from the pdf file with further geographical information on the study area. I am a total QGIS noob and I don't know if what I want to do is possible at all.

What I have done so far is: I copied, digitized and and georeferenced the map contained in the pdf. I then set the CRS according to the information I got from the pdf. I then tried to "reproduce" the grid with 16km cells using the QGIS vector grid tool.

Now, here's my problem: the side length I enter into the vector grid tool is in degrees not km. I have measured the length of the grids in degrees (using the QGis distance measurement tool). If I try to create the vector grid using the degree measure for the side length I get cells that are approx. the size of the original map at some parts of the map but not at others. Finally, even if I got a grid of the same size as the original, I am not quite sure how to adjust the location of the grid I created to the exact same location as the original one.

Answer 1

Most probably your project CRS is set to default WGS84, that is lat/lon degrees.

You better set the project CRS to that of the Geotiff you created. Then the values for vector grid are in the units of the CRS. You can select min and max values to the ones you need, either by taking the values written on the map, or moving the mouse to the lines and noting the coordinates from the infobox.

And make sure to create polygons, not lines. Otherwise your grid gets skewed up if you wish to change the project CRS.