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

[Raymond Nijssen]

Hi Tad,

I'd say you should save your data in a local projection first. Then you 
can use meters as units which will make more sense than 0.000000001 
degrees or whatever.

I tried 10 meters in epsg:3857 and it worked out well.

Good luck,
Raymond

On 08-01-18 18:26, Tad Deshler wrote:
> 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 
> <mailto:tad.deshler at cohoenvironmental.com>//
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