ETOPO1: Distinguishing between negative elevation and bathymetry

[j0nes2k]

I am trying to get elevation information at this point:
https://api.opentopodata.org/v1/etopo1?locations=38.6920,2.7012

When using srtm30m, I get null values. Therefore I was looking at replacement models and came across ETOPO1. When using this dataset as per the URL above, I get an output of -1926.0. This makes sense, as this model should include both bathymetry as well as elevation.

But: how can I distinguish between those two? There are places on earth with negative elevation, and bathymetry by definition is always negative.

[ajnisbet]

Good question!

Yes, for a model that includes bathymetry, you can't be sure if negative values are land or ocean. As well as the elevation data, you need a second channel that is a water/land mask, and I'm not aware of any open global datasets that include both bathymetry and water mask.

You can use a second dataset to be the water mask. For example the Copernicus DEM comes with a raster water body mask (WBM) dataset that can be added to a self-hosted opentopodata instance. Then you could query both etopo1 for the elevation and Copernicus DEM WBM for the land/bathy data. Or you could use a coastline vector to classify land points.

The downside with mixing datasets (especially with differing resolutions) is that they may differ in their classification of the same point. For example, with the 30m resolution Copermicus water mask or the 10m resolution coastline vector, a point 500m offshore will reliably be classified as bathymetry. But due to etopo1's 1km resolution, that same point may have a positive elevation still! You may want to use some approximation like: if a point is classified as bathymetry, but has a positive ETOPO1 elevation, set the elevation to 0.

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I also run GPXZ which does have a bathymetry toggle for point queries: https://www.gpxz.io/docs/api-reference-points#elevation-points

If you're working on a personal or academic project I'd be happy to give you a free account.