Serving Lidar Data - Under the Hood

 

Mosaic Datasets | Aspect | Hillshade | Shaded Relief | Slope

 

There are multiple lidar-based DEM datasets on CT ECO. Each dataset consists of multiple tiles of raster data (meaning they consist of pixels). Mosaic datasets are an Esri format used to bring the tiles together into one, seamless DEM layer. From there, processes called functions are applied so that the elevation can be visualized in different ways. On CT ECO, you can view the 2016 DEM with functions already applied in the elevation viewer and as image services. To being with the DEM service and apply the functions yourself, visit the Elevation Interact map or connect to DEM image services.

Mosaic Datasets

Esri's mosaic dataset format is used extensively to serve large amounts of raster data. A mosaic dataset is a model within the geodatabase used to manage a collection of raster datasets (images) stored as a catalog and viewed as a mosaicked image.

First, a mosaic dataset was created containing all 23,381 DEM tiles from the 2016 flight. Five reference mosaic datasets were created from the original mosaic dataset. A different function or set of functions was applied to each reference mosaic dataset to create the desired reference mosaic datasets output which eventually because the image services.

Mosaic Dataset

 

Reference Mosaic Datasets

 

DEM

(Digital Elevation Model)

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Elevation

Hillshade

Slope

Aspect

Shaded Relief

 

Functions

Aspect

Aspect identifies the downslope direction of the maximum rate of change in value from each cell to its neighbors. In other words, the direction the slope faces. Aspect is reported in degrees of a circle where 0° or 360° is north, 90° is east, 180° is south and 270° is west.

Aspect function details.

Aspect Explained

Hillshade

The hillshade function is applied to the hillshade reference mosaic dataset that comes from the derived mosaic dataset.

Azimuth: 315, Altitude: 45, Scaling: none, Z factor: 2.

The output is an 8 bit unsigned raster.

More on the hillshade function and how hillshade works.

 

Shaded Relief

The shaded relief function is applied to the shaded relief reference mosaic dataset that comes from the derived mosaic dataset.

Azimuth: 315, Altitude: 45, Scaling: none, Z factor: 2.

The output is an 8 bit unsigned raster.

More on the shaded relief function.

 

Slope

The slope function is applied to the slope reference mosaic dataset that comes from the derived mosaic dataset. The output is DEGREE and a Z factor of 2 is applied.

A minimum-maximum stretch (0 to 90) with dynamic range adjustment is also applied to the output to improve the contrast while viewing.

More on the slope function.