# how to make decent sense from scattered river depth measurements

I sampled river depth at scattered positions and I want to be able to make a map and GIS layers out of it.

My problems are:

1. As the river meanders the interpolation I do in scientific packages i,e. matlab generates values in points where there is no river therefore when I create a KML file out of it, it covers land areas. Does QGIS/GRASS have a better solution for that?

2. My GPS points accuracy of ~3m 90% of the time. Do I need to do any filtering for location?

• Searching our site for interpolation + barrier turns up some relevant threads. – whuber Oct 29 '13 at 14:17
• Interpolating is a pretty simple process. In most cases interpolation is linear, though it can be done to suite any sort of curve that can be functionally defined. If you choose to define your function as the set of related coordinates that currently define your river (say, by you drawing a curve that represents the river), then it is a simple matter of associating that function with the points you have sampled. I'm going to say that Matlab is not yet sophisticated enough to interpolate such a complex shape without a little more help from you. Add a few false points to help Matlab. – ike Oct 29 '13 at 16:11
• For interpolating a meandering river, you should use a flow oriented coordinate system. You can transform your data along the centerline of the river in Matlab using a method such as Merwade's "S-N" coordinate transformation. An implementation of that method is the xy2sn function, which is available on the Matlab Fileexchange. – Jake Oct 30 '13 at 8:56
• If you gps is ~3m for horizontal accuracy, the vertical accuracy is probably +- 10 m. What did you want to do with the elevation data? Water typically flows at 1 m fall for every 10 000 m distance. So handheld gps accuracies are not useful for hydrological work. – Willy Oct 30 '13 at 10:26
• With regard interpolation on a meandering feature, the most practical solution is to increase the number of samples. – Willy Oct 30 '13 at 10:29