# Building a DEM with both a raster and vector points [closed]

So I'm struggling here, trying to get a decent DEM for the Azores Islands because I want to include bathymetric data on it.

I have a raster with elevation for the islands (land) with a 30 meter resolution, which is actually more detail than I would ask for but, unfortunately, the bathymetry I have managed to acquire are vector points 3000 meters apart.

I converted the raster into contour lines and tried to make an interpolation using both elements (points and lines), needless to say I run into problems around sea level.

My best results were obtained using a spline interpolation with different smoothing for lines and points on GRASS, but I get artifacts whenever the distance between a vector and a line is close to 3000 meters.

Any tips on how to get a decent DEM when using data of different types and with such different resolutions?

## closed as unclear what you're asking by sig_renato, Dan C, Andre Silva, John Powell, PolyGeo♦Feb 15 '16 at 22:24

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• Since you want to end up with a raster DEM, there is no need to convert to contours prior. You should be able to create a DEM from your 3000 meter bathymetry. Since that is your lowest resolution data, you will need to convert the 30 meter elevation DEM to a 3000 meter grid. You should then be able to combine them together. Some things to think about are making sure they are the same projection, and that the grid is using the same x and y values. What I mean there, is if one is numbered 0, 30, 60 and the other is 10, 40, 70, some interpolation will be required. – Get Spatial Jan 25 '16 at 23:36
• I should mention that the Azores are volcanic islands, I can't afford to use a 3000 meter resolution for the terrain, some of the islands are not even 5 km wide! I wanted to get a DEM combining available resolutions. – sig_renato Jan 25 '16 at 23:49
• You have a valid point. Given that, though, it begs the question of what your intended use of this layer is? That is a huge gap in resolution that one way or another is going to cause trade-offs in how you can use it in the end. If it is simply for illustrative purposes, is it worth combining them at all, or simply leaving them separate? – Get Spatial Jan 25 '16 at 23:53
• Your last sentence in your last comment illustrates the entire problem. You are not going to be able to get decent bathymetry by combining a line indicating sea level, with a 3000 meter grid bathymetric layer. There is simply not enough resolution in that layer to give any detail related to underwater fishing pockets and the like. If you want that sort of detail, you need to find a higher resolution source of bathymetric data. No spline interpolation or other form of creating contours will solve the underlying problem of lack of resolution in your source data. – Get Spatial Jan 26 '16 at 0:14
• I am not sure where you have been looking, but you might check out this resource: General Bathymetric Chart of the Oceans It looks to be 30 arc second coverage which is significantly better than 3000 meters. It still won't mesh well with a 30 meter grid, but will at least show you some more detail. – Get Spatial Jan 26 '16 at 0:33