[Qgis-user] georeferencing

[Michael Dufty]

I’ve found if you tick “Use 0 for transparency when needed” in the transformation settings, before you georeference, the bands you mention will automatically be transparent.
This only seems to be necessary if you start with a jpeg.
If you start with a tif or png format image, that supports transparency, the resulting image has transparent borders regardless of the setting.
This is probably the same as Nicolas’ method, just that qgis does it for you.  Nicolas’ method is more useful if you already did the georeferencing and have not saved GCPs to do it again.

For clipping – look in the raster menu – Extraction, there are two options, clip to extent or clip to mask layer.

Another useful trick with scanned maps is, if you have scanned a line drawing type map that is predominantly white, try setting rendering mode to multiply – this makes white areas completely transparent so allows you to overlay the scanned map over other data, photos etc. very easily.


From: Qgis-user <qgis-user-bounces at lists.osgeo.org> On Behalf Of Firstname Lastname
Sent: Friday, 3 September 2021 1:39 AM
To: qgis-user at lists.osgeo.org
Subject: [Qgis-user] georeferencing

i have successfully georreferenced several maps in QGIS.  The georeferencing tool leaves a black band around the rotated and shifted image.  is there any way to get rid of this.  also, i would like to clip some  of these images to get rid of the margin material.  is there a way to do this in QGIS?
--
Byron Veilleux, MSc. P.Geo
Conjugate Geologic Services Limited

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