[Qgis-user] Interpolated water depths overlap poorly with underlying sample points
Raymond Nijssen
r.nijssen at terglobo.nl
Mon Jan 8 09:52:58 PST 2018
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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>
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>
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