[Qgis-user] Interpolated water depths overlap poorly with underlying sample points

[Tad Deshler]

I have created a vector layer from a spreadsheet that contains water depths
and x-y coordinates in latitude and longitude (276 points in all, see
attached). I've used the Interpolation plugin to create a TIN of water
depths, but the interpolation doesn't cover the same region as the
underlying points (see attached graphic). The results I do get seem accurate
(deeper red points are at southern edge of grid), but are only partially
what I need. 

After specifying the interpolation attribute (Depth), the output section of
the interpolation plugin displays the default number of columns and rows
(300 for each). The Cellsize X is specified as 0.00001 (I think because my
longitude data is to five decimal places). So I left number of columns and
Cellsize X as 300 and 0.00001, respectively. The number of rows is also
specified as 300, but the Cellsize Y was initially specified as 0.00000,
which not surprisingly yielded nothing for the interpolation. So next time,
I changed the Cellsize Y value to 0.00001 to match Cellsize X. This
automatically changed the number of rows to 139. Could this be because the
spread of my data is from 61.1201 to 61.1215 degrees North, which would be
140 times 0.00001? 

I'm using QGIS 2.18.11. I am confused about why the interpolation doesn't
cover all the sample points. I have tried changing the values for X and Y
min and max (making them larger than the actual data range), but this only
serves to place the interpolated result OUTSIDE the sample points, so I
don't think that is the solution either. 

Can anybody provide some guidance on where I'm going wrong here?

 


Tad Deshler

Coho Environmental LLC

206.778.9274

tad.deshler at cohoenvironmental.com

cohoenvironmental.com

Logo-very small

 

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