Retrieving State Heightmaps (DMTED) - Accuracy possible with QGIS? - Requesting Steps

Question

I need a gmted2010 height map of Massachusetts. Only Massachusetts, including its shoreline, islands, and of course state boundaries.

What I am currently doing, in bullet point form:

I am using https://earthexplorer.usgs.gov/ - this is where I select a predefined area to get Mass. Though it sounds like I will get the option to download a Mass heightmap, I get the entire New England area + more.

Once I have options to download the heightmap, I have selected the 7.5 arc.

I input all heightmap files into QGIS, where they all layer on top of one another.

I retrieve the Shapefile from Mass.gov - I am using the 25k Polygon for the stat of MA.

I select the Open Datasource Manager and select Vector and upload the 25k polygon. In the processing toolbox I select clip raster by mask layer.

Once I get the "final image" I am left with an image that does not follow the border around states, the shoreline, and outlying islands. Rather I get the raster effect. I have added two pictures below so you can see what's going wrong.

Essentially, I need the heightmap of Massachusetts. I am making some mistakes along the way. I think it might have to do with how the image from USGS is a raster, and I am using a polygon in Vector form.

In other words, the idea is that the heightmap should be identical to the mask. I should be able to export a high quality TIFF of that heightmap.

Would someone be able to provide a step by step process of how I might be able to get a heightmap of Massachusetts? (or other state or select location, and not have parts/area missing through raster in my "final image."

Answer 1

"...I am left with an image that does not follow the border around states, the shoreline, and outlying islands. Rather I get the raster effect."

DEM data is raster data, so you will naturally (and correctly!) end up with "...the raster effect."

Because of the DEM's inherent raster nature, you can only get the pixelated, stair-step look with such data. It will never exactly follow the non-orthogonal angles that vector data can display (such as your coastline). However, you can more closely approximate such vector boundaries by sourcing raster data with a smaller pixel size, which in turn reduce (but not eliminate) the raster effect.