[Qgis-user] Georeferencing issues and vector grid problems

[Nicolas Cadieux]

Hi 
See below for answers,

> Le 16 nov. 2016 à 05:00, Alexander James Metcalfe [via OSGeo.org] <ml-node+s1560n5295988h89 at n6.nabble.com> a écrit :
> 
> 
> Hi QGIS community, 
> 
> My reason for using QGIS is relatively simple. I could really use some help to set me straight and I would be very grateful.
> 
> My study area is Duncan Down in Whitstable, UK. I have attempted to geo-reference a hi-res image of this site from Google Earth. 
> I want to over lay this with a grid to represent quadrats for vegetation mapping and random sampling. 
> I'll use images taken from these stages to include in  my methods section of my MSc dissertation/thesis
> 
> My problems are: 
> 
> That, in georeferencing the google earth image (with the georeferencing tool) I have been able to get it to line up  just about right with the openlayers google earth plugin visualisation. I do this by entering coordinates gained from google earth manually into the georeferencer. 
I hope you are using the Google Earth Image just for visualisation.  They are not good for any kind of spectral analysis.


> Unfortunately when I go to create a vector grid (via research tools) a pop-up tells me the CRS for the raster (I am creating the grid for, the study site) and the project are different. I can get it to create a grid but it is always rectangular rather than square. I understand that this might be a result of on-the-fly CRS transformation. I'm not sure whether this is somehow incorrect or will affect my ability to properly map GPS points gained in the field work. 

Yes, if your project is in long /lat, the grid will be a rectangle and not a square unless you are on the equator.
> The project is WG 84 / Pseudo Mercator EPSG: 3857 and the raster is WG 84 / Pseudo Mercator EPSG: 4326. When they are like this they are aligned. When I change the CRS for the raster to match the project (e.g. from 4326 to 3857)  it ends up being transported to the somewhere else off the coast of South America! And, it is very skewed. 
You cannot just change the CRS.  You need to reproject the raster.  To do that, use gdal warp in the raster menu.  Just changing the crs in the layer is only good if you have made an error and want to correct that (for example, change the utm zone).
> 
> 
> I feel like I am trying to do a simple thing here and it's going wrong at every turn....
> 
> Kind and hopeful regards, 
> 
> Alex
> 
> 
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