Working on a map that includes a bathymetry layer, SRTM data for hillshade, and an accurate coastline boundary edge. I noticed though that the pixel resolution of SRTM 90m resolution creates visible blocky edges when zoomed in (this screenshot equates to z12/13 in web mercator zoom levels).

enter image description here

This is in Alaska, approx. latitude 57°N.

Are there any cartography or editing techniques I could use to clean this up?

Either in clipping the data or in smoothly transitioning the edges to the coastline?

Another example dataset (not SRTM but a DTM of the same area) overlaps outside the coastline edge, similar problem:

enter image description here

  • 1
    Please Edit the question to specify the latitude. SRTM changes pixel width as you approach the poles, and this often manifests as repeated pixel values.
    – Vince
    Commented Jan 2, 2019 at 13:59
  • Added, forgot to put it there in the original post!
    – colemanm
    Commented Jan 2, 2019 at 15:20
  • I would create the hillshade based on the full extent of the DEM, rather than a clipped version. Then overlay your bathymetry layer and a clean border.
    – Aaron
    Commented Jan 3, 2019 at 16:38
  • If you just want to smooth the boundaries of the SRTM data, I would convert it to a binary image (ocean should be 0, land 1), then pass a smoothing kernel over the image. Finally, I'd just add the SRTM to the mask, which will effectively fill in the rough portions of the shoreline with the smooth version.
    – Jon
    Commented Mar 5, 2019 at 22:18

2 Answers 2


I would say as the underlying data is a raster and therefore "blocky" if you are looking for a smooth edge you need to convert your raster into a smoothed polygon. All GIS systems allow you to convert from raster to polygon and generalise the edge. If you want to retain the hillshade then I guess you need to create a polygon dataset with a hole that is the coast and mask out the sea and layer appropriately. Well that's how I would have done it in ArcMap.


There are better sources than SRTM Even vector data that's available. I have vector tiles of world shoreline and international boundaries.


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