Alaska Elevation Portal Help

Tools

	Select a point on the map to view a list of project-level data available for download. Using this tool will result in the download of the full extent of the project data. Use the Area of Interest tool to select a smaller subset of the layer to reduce file size. Several lidar projects may overlap and not all available layers are checked for download by default.
	Draw a rectangle around your area of interest to view a list of tiled data available for download. This option will allow you to download only the data you need. Use the Select Project Data tool to view the list of available projects in their full extent, which will result in larger download files. Several lidar projects may overlap and not all available layers are checked for download by default.
	Hover over this icon to select different base maps or overlays - including PLSS and Quadrangles.

Navigation Tips

Holding the shift key while drawing a rectangle with the mouse will automatically zoom to those extents.

Lidar Terminology

Lidar

Lidar is an acronym for Light Detection and Ranging. By pointing a laser at a surface at closely spaced intervals and measuring the time it takes for light to return to the source, very accurate three-dimensional information can be obtained with sufficient measurements. Billions of these measurements can yield an extremely detailed three-dimensional rendering of the terrain.

Hillshade

A hillshade is a grayscale rendering of the terrain which uses a simulated sun angle to shade and model the terrain features. A hillshade appears more like the terrain you might view from an airplane.

DEM

A DEM is an acronym for Digital Elevation Model. Similar to a digital image where individual pixels contain red, green, and blue values to describe the color of a feature at that location in space, a DEM pixel contains the height of the elevation at that location.

DSM

A DSM is an acronym for Digital Surface Model. This is a type of DEM that specifically describes the “top surface” elevation. If a loose blanket were draped over the topography, the DSM would represent the elevations of the topography taking into account trees, buildings, and towers.

DTM

A DTM is an acronym for Digital Terrain Model. This is a type of DEM that specifically describes the elevation of the ground surface, or a “bare earth” model. If all of the trees, buildings, vegetation and towers could be removed leaving only the bare ground beneath, the DTM would represent that topography.

Point Cloud

A LiDAR sensor sends out several light pulses per second and receives multiple returns from different objects from each pulse. The calculated position of a return is called a point. By calculating the position of every return, a “point cloud” can be derived to show the positions of all returns.

Point Density

Point density is a measurement of the number of points (returns) per unit of ground distance. For example, if you counted the number of points in a point cloud within one square meter, the number would represent the point density. Or, if there are 10 points within a square meter in the point cloud, the density is “10 points per square meter”.

First Return

When the sensor sends out a pulse of light, that light may illuminate multiple objects on the ground. The light then bounces off those objects and is returned to the sensor as returns or “points”. The First Return is the first measurement to be received back by the sensor. Often, it might represent what the light pulse hit first, or what is tallest in the scene, such as a tree. First returns are also used to generate a Digital Surface Model.

Last Return

Similar to first return, the Last Return is the last measurement to be received back by the sensor from a single pulse of light. Often, it represents the last object the light pulse hit, or what is furthest away in the scene, such as the ground surface. Last returns are used to generate Digital Terrain Models.

Resolution

Resolution typically refers to the pixel size of an image. For example, a 3-foot resolution DEM means that each pixel represents a 3-foot by 3-foot cell on the ground. This is important information when working with DEMs because it helps define the level of detail that can be resolved in the scene. A 30-foot resolution DEM may only distinguish large buildings and features, whereas a 3-foot resolution DEM may identify smaller buildings, road surfaces and individual trees.

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