Data Definitions

Derivatives:

Datasets in ArcGrid format are available for selected Illinois counties. The original, classified LiDAR data point cloud has been processed to create a digital terrain model (DTM) and a digital surface model (DSM) for each county. Additionally, each county .zip file contains an associated shaded relief model, or ‘hillshade’, an ArcGIS v.10.1 .mxd file, and also ArcGrid pyramid files and statistics to facilitate display and analysis. This zip file includes the LiDAR data point cloud processed into created digital terrain models (DTM) and a digital surface models (DSM) for each county. Additionally, each county .zip file contains an associated shaded relief model, or ‘hillshade’, an ArcGIS v.10.1 .mxd file, and also ArcGrid pyramid files and statistics to facilitate display and analysis.

Digital Surface Model (DSM)

Digital Terrain Model (DTM)

Digital Elevation Model (DEM)

LAS Point Cloud

DSM: all returns
Processing all returns in the classified point cloud that represent both the ground surface and aboveground features (buildings, roadways and overpasses, vegetation, etc.) produces a DSM. A DSM is commonly referred to as the ‘First Returns’ reflective surface model.

DTM: ground returns with hydrologic breaklines
When only the ground returns in a classified point cloud are used, the resulting hydro-flattened grid of bare-earth elevations is a DTM. Also, lake surfaces are smoothed and stream courses are conditioned for downhill flow using breakline data. This subset of the processed returns from the airborne laser sensor represent the ground surface and is commonly referred to as the ‘Bare Earth’ point cloud.

DEM: ground returns without hydrologic breaklines
When hydro-flattening is not imposed during processing of the ground returns, either because of missing or insufficient breakline data, the resulting Bare Earth elevation grid is classified as a DEM.

LAS
The LAS file format is a public file format for viewing the LiDAR point cloud data.  Each data point is recorded with precise horizontal position, vertical elevation, and other attribute values. 

Metadata
This is a file that describes the content, quality, condition, origin and other characteristics of the LiDAR data collection. The Metadata file also includes the appropriate citation to use when citing the Lidar data.

Tile Index
This is a shapefile of the point cloud tile boundaries with tile name.  This file is helpful to view if you are interested in a smaller subset of the data to select specific tile names.

Breaklines
These are the hydrological boundary lines that define streams, shorelines, and dams. These are a linear GIS feature that describes a change in the smoothness or continuity of a surface. These allow us to create Hydro –enforced Digital Terrain Models (DTM).

DAT/DGN/TIn Available
This column will be checked off if there are available DAT,DGN or TIN files available for this collection.