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rachel.jensen

Grid Handling Tips in QGIS

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Q:

I have a QGIS query and hopefully you can easily answer it.  I know how to do this with my eyes closed in MI, but, I was wondering if you have any tips for:

•    merging a grid (by grid order) in QGIS (layer stack merge never seems to work), and;
•    exporting a grid to .flt in QGIS

I want to “read grid zpts” the grid(s) so if you don’t have a solution for the layer stack merging I can read them in order if I can export as .flt – I find ASCII’s produced by QGIS don’t behave like those created in MapInfo, do you also find this?

 

A:

If you want to merge a grid, I usually use raster>miscellaneous>merge. There is a useful tute here.
I normally use this for adjacent grids, where they don’t overlap.
If you need to get a bit more in depth then the SAGA tool box is the way to go.
This stack exchange post is pretty much what I use. You can use first  or last method to make sure that you get the right grid order.

Exporting to flt is a bit trickier. QGIS currently does not export these perfectly. You can export as the ESRI BIL file and change the header. Here’s an example.
Another option is to export as asci in QGIS then use the TUFLOW utility asc_to_asc to convert it.

I usually use TUFLOW to read in the grids in the correct order, then typically work with the resulting DEM_Z.flt file.


Sometimes, QGIS can default to the ‘golden surfer’ style of asci, which doesn’t have a fixed cell size. This may be why you’re seeing a difference. The ‘export asci in QGIS’ link above, there’s instructions on how to get around this.
 

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