[Qgis-user] point pattern analysis in West Africa

[Nicolas Cadieux]

Hi,

Thanks for sending me the files. I will respond to the mailing list so 
everyone can see. This is very easy to fix. The files are in WGS84/UTM 
ZONE 31N (epsg: 32631) (the unit for this is meters).  The files 
_should_ be in WGS84 longitude latitude EPSG 4326.   I am working in 
QGIS 2.18 for this example.

1) Start with an empty project.  Make sure it's in epsg: 32631. Load 
your point file (Final modified CV locations after Akom (UTM31N).shp) 
and zoom to the file extent.  Look at the Coordinates at the bottom of 
the screen.  You will see values such as 6.70, 5.79.  These are 
latitudes  and longitude values but you project CRS and file CRS are in 
meters.  Values should be in the thousand and millions like 
(-252932,449628).  Look at the file database.  You will see Longitude 
and Latitude.

2) Install QuickMapServices plugin.  You will now have a Web menu.  Go 
to Web>QuickMapServices>Settings>More services>Get Contributed pack.  
Now load a Map Background like google earth.  This background will help 
you see the error.  Your project EPSG) code will probably change to 
EPSG: 3857.  Change that to 4326 (WGS84 Latitude/Longitude.)

3) Zoom to both file extents. (layer panel>zoom to layer)  You will see 
that you points are in the ocean.  Unless Ghana has moved recently, this 
is the wrong place for you points. (sometimes, Qgis or the plugin will 
crash or send an error message when flipping to erroneous coordinates.  
Don't worry.

4)You will need rewrite the file projection.  Select the point file 
(Final modified CV locations after Akom (UTM31N).shp) and in the layer 
panel right click to "Set layer SRC" and select EPSG 4326.  Zoom to the 
layer and you will now be in Ghana.  You then need to save the file in 
the correct EPSG code.  (save as).  Do NOT use gdal_warp or change the 
CRS in the save menu.  This will reproject the file but keep it in the 
wrong place.

5) Do this with all your files.  You can batch process this if you have 
too many files.

If you want to create a grid that is exactly 100kmX100Km, you will need 
to create that file in EPSG 32631 as that CRS is in meters.  If you need 
to figure out your file extent, take a corrected file in EPSG 4326 and 
reproject it in 32631 and use that file to find the grid extent.

Hope this helps!

Nicolas



On 2018-09-17 10:27 PM, RMG wrote:
> Hi,
> I have a species localities over four countries in West Africa, mostly 
> straddling Ghana. I have set the layers' CRS to WGS84 UTM31N. I want 
> to do point pattern analysis by overlaying a grid system over the 
> species range. I created a 100 km x 100 km grid layer (MMQGIS), but 
> when I measure each length of the grid using the measurement tool, it 
> shows up as 1 km. What am I doing wrong? If I change the project CRS 
> to Clark 1880, the distance unit correctly shows up, but it measures 
> 180 km. What is the remedy to this problem?
>
> Thanks for your attention.
>
> Reiko M. Goodwin
> facebook.com/GuenonConservationCommunity/ 
> <http://facebook.com/GuenonConservationCommunity/>
>
>
>
>
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