DAV Tips: DEMs or Points


The latest version of the Data Access Viewer (DAV) has been our for about a year now and we’ve learned a few spots where people have trouble. In this post, I’ll look at an issue that may cause confusion because it is not at all obvious – lidar points versus digital elevation models (DEM). We used to only store the lidar point clouds and we’d derive the DEMs from the points upon request. However, you may now find two datasets with almost identical names, one stored as a point cloud and one stored as a raster DEM, and both can give you a DEM. They each have pros and cons and your choices will differ, which isn’t obvious when you’re picking one.

How to Tell Points from DEMs

This is actually quite easy and you may have already picked up on it. The DEM data sets all have either DEM or Digital Elevation Model in the title. The point clouds do not. Simple as that.

 

NC DAV Choices
Example of search results with a point cloud (top) and a DEM (bottom) for the same area. Note that the point cloud with over 4.5 billion points in that area is three times the current 1.5 billion point limit and can’t be added to the cart. The DEM is well within the 20 Gb limit at only a bit over 8 Gb.

Advantages of DEMs

Starting from the DEMs gives you a several clear advantages over the points.

  • Faster to process since the raster is already made and you might only need to reproject.
  • The size limits are more lenient, so you can grab a bigger area.
  • The DEMs probably had some manual work and breaklines applied for a cleaner product that is ready to use.
  • The DEMs along the coast have the option to apply VDatum to get a tidal datum. Naturally, this will limit the data returns to only the area where the VDatum grids are valid, so it doesn’t go very far inland.

Advantages of Points

Starting from the point cloud gives you many more options currently. Obviously, you can only get the point cloud if you start with points and not a derived product like the DEM. Here are some other advantages:

  • Create the DEM by first projecting the points into the desired projection. Reprojecting an existing DEM will always cause some sort of distortion and/or interpolation. Starting from the points avoids that.
  • The DAV can create contours from the points. Doing this from the DEM is certainly a possibility in the future, I just haven’t added it yet.
  • The DAV gives you options over the interpolation methods for creating a DEM and what point types are included. This can be particularly useful if you need a class that wasn’t included in the existing DEM, such as bridge decks, or if you want to see where the void areas are.

Both?

Of course, sometimes you need to get both so you can see multiple things. Just add them both to the cart. Not all data sets have points and DEMs available as separate datasets, but we’re trying to get both up for new datasets we receive. If there is demand for some of the older datasets, we’ll try to make them available too. I’m hoping to add a series of DAV tips, so if there is something you’d like discussed, please let us know.

Kirk Waters

I’m a physical scientist at the NOAA Office for Coastal Management. In my spare time, when I’m not torturing co-workers, I try to fit in some technical work on lidar processing and distribution. I also try to figure out ways to improve the Digital Coast’s data offerings in general. Somewhere in the back of my head there are still a few brain cells that remember satellite ocean color, oceanographic field work, and something about the ozone hole.

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