QGIS Georeferencer quetion

[vorropohaiah]

If I'm not wrong (I've not read everything, I must confess), you could just use Gprojector. It's using equirectangular projections to turn them into the projection you want (there's a wide range of them).

Available here.

G. Projector works but it exports in ridiculously low res and you can only upload pretty low res files in the first place, which is a shame as it would be good, otherwise

[Rwhyte]

...I'm drawing an equirectangular map and have imported it into the QGIS Georeferencer, entered the co-ordinates....

Just curious, what are the coordinate values that you enter when placing the map in the Georeferencer? This may not be your workflow exactly, but may offer some clues on what's happening in QGIS...

If you're entering Geographic coordinates (eg Lat \ Long in degrees), as in -180, -90, 180, 90
georef_pts.JPG

Then you'll want set the target Spatial Reference to WGS 84. This is to say, it could be difficult to know what coordinate values to enter to georeference a map directly into projected coordinate system, unless you happen to already have some known points on hand... Every projected CS has a different range of values, often very large values, that are not always intuitive (compared to lat, long).
Geored_transform.JPG

Using WGS 84 would georeference your layer with geographic coordinates. And it happens to be displayed in QGIS as equirectangular, since that's the default way to display a layer with a geographic coordinate system
GeoRef_WGS84.JPG

Once it's been georeferenced this way, using the Warp tool (as mentioned above) you can then project the layer as a new layer in a projected coordinate system. Even if you get errors, the tool may have worked. However, (and here's the kicker) if you view that layer in your same map project, maybe it appears just the same

Hey, what gives? It looks just the same? Keep in mind that a QGIS project will apply the current map's existing coordinate system to any new layers added, it will 'project on the fly' to keep all the layers aligned. (Often this is really convenient, except here, when it seems to undo what you're after).
GeoRef_inQGIS.JPG


If you happen to view the projected tif image Outside of QGIS, as an image in your file system, it should look projected (I think as shown in the previous post about using Warp). If you start a blank, new QGIS project and add this as the first layer, it'll appear using the layer's own projected CS.
GdalWarp_export.JPG



As a shortcut, (using this same project on the fly component ). If you have your original georefereneced 'geographic coordinates' layer, and switch your map project's coordinate system,QGIS--> Project --> Properties --> And set the Map Project's Coordinate System to a Projected CS, eg North America Albers Equal Area
QGIS_CRS.JPG

You'd end up with the projected view... It will also probably throw some errors, but this is all pretty general, to you can probably ignore.
prj_on_the_fly.JPG

Anyways, hope some of that may help, in the ever-unfolding saga that is understanding coordinate systems, and their quirks in GIS.

[vorropohaiah]

thanks for that, I managed to get it to work, though it seems a bit moot though as the reprojected image is such low resolution its a waste of time.

[Repporio]

You should be able to decide your own resolution when exporting the image. (Layer->Save as...)

[vorropohaiah]

You should be able to decide your own resolution when exporting the image. (Layer->Save as...)

I actually was not aware that you could save like that. I was ding Project --> export --> export map to image. but i tired layer --> save as and none of the available formats were formats I can work in wth in PS. I tried saving asa GeoTIFF, hping it was like a normal TIFF file but I got an error when saving.

[vorropohaiah]

You should be able to decide your own resolution when exporting the image. (Layer->Save as...)

I actually was not aware that you could save like that. I was ding Project --> export --> export map to image. but i tired layer --> save as and none of the available formats were formats I can work in with in PS. I tried saving as a GeoTIFF, hping it was like a normal TIFF file but I got an error when saving.

[AzureWings]

When I've exported map images from QGIS I've always used the Print Composer (Project > New Print Layout..., or Project > Layouts > [your layout name] once you've created it). Once you've placed a map object in the composer, when you refresh it it will show the current map displayed in the editor filling the space you've defined on the canvas (which could be the entire canvas). You can select the coordinate system to export in by selecting the map object in the canvas and going to "Item Properties" on the right, which lets you pick the exported CRS (as well as the displayed map extents and a bunch of other settings). Finally, when you select "Export as Image" in the Print Composer's topbar after selecting the filetype and name to save you'll be able to enter the export resolution (at least for PNGs, which are all I've used it for).