Connecticut Lidar Frequently Asked Questions

 

Have a question? Email us at clear@uconn.edu.

 

What is lidar?

Is it called LiDAR or LIDAR or Lidar or lidar?

What is the coordinate system and units?

How is so much lidar data being served so fast?

How do I download the data?

What is the Quality Level (QL) of each dataset?

What is a DEM?

What is hillshade, shaded, relief, slope and aspect? See the Elevation Explained page.

What is the difference between Shaded Relief (dynamic color) and Shaded Relief?

What do the numbers on the ASPECT map mean?

What do the numbers on the SLOPE map mean?

What is azimuth and altitude?

 

What is lidar?

Light detection and ranging (lidar) is a rapidly evolving remote-sensing technology that uses a laser to measure distances (ranges) between the sensor and the target(s). This ranging information is converted to three-dimensional information that can be used for a wide range of applications (USGS).

Lidar is a remote sensing method used to examine the surface of the Earth. What is Lidar? from NOAA

Lidar is an optical remote-sensing technique that uses laser light to densely sample the surface of the earth, producing highly accurate x,y,z measurements What is Lidar data? from Esri

 

Is it called LiDAR or LIDAR or Lidar or lidar?

According to USGS, lidar is a term formed by combining more than the first letter of common words and therefore is set in lowercase, similar to the terms "sonar" and "radar" (GPO Style Manual 2008, 9.48, p. 235). Lidar is not a product, a proper name, a word copyrighted or owned by anyone, nor is it a law. Vendors who create the software that use this technology may label their software HDL-64E LiDAR or ALS50 Airborne LIDAR as a product name. If lidar technology is referred to in a general sense, light detection and ranging should be spelled out at first use and followed by the abbreviation as follows: light detection and ranging (lidar).

 

What is the coordinate system and units?

Although the original coordinate system of DEM tiles vary, the elevation image services are all in the coordinate system WGS 1984 Web Mercator Auxiliary Sphere, which is the current standard for web services ( EPSG 3857 formerly 102100). The elevation, or z values, are all in NAVD88. NAVD stands for North American Vertical Datum of 1988. Read more about it here and here. The 2016 dataset z values are in NAVD88 feet where other datasets have z values in meters. The Lidar Info page lists the originating coordinate systems for all elevation datasets on CT ECO.

 

How is so much Lidar data being served so fast?

Visit the Under the Hood page for a thorough description. In a nutshell, Esri's mosaic dataset is a way to manage large amounts of data that can be published through ArcGIS server. It uses techniques such as referencing data and creating overview images to decrease the amount of data that has to draw each time the view is changed.

 

How do I download the data?

The simplest way to use the data is to connect to the image service(s). Detailed instructions are available for connecting using ArcGIS Online, Google Earth and ArcMap as well as links to the REST endpoints. If you are looking for data files that can be downloaded, stored on your computer and manipulated, the LAS files are available for download through the NOAA Office for Coastal Management using either the Data Access Viewer or the Data Registry. The 2016 DEM tiles can be downloaded on CT ECO.

 

What do I do if something looks wrong?

Let us know. Contact information is at the bottom of the page. If at all possible, please describe what is happening and where. The coordinates of the location are easy to find in the Elevation Viewer using the coordinates tool.

 

What is the Quality Level (QL) of each dataset?

Quality levels as defined by the National Enhanced Elevation Assessment (NEEA) of the USGS. Further explanation here.

All data for Connecticut are QL2 except for the two New England Lidar Datasets (East and West) which are QL3.

Elevation Quality
Levels (QL)
Source Point Density Nominal Pulse
Spacing (NPS)
DEM Post
Spacing
Vertical
RMSEz
Equivalent Contour
Accuracy
QL 1 Lidar 8 pts/m 0.35 m2 ∼1m 9.25 cm2 1 ft
QL 2 Lidar 2 pts/m2 0.7 m2 ∼1m 9.25 cm2 1 ft
QL 3 Lidar 1-0.25 pts/m2 1-2 m2 ∼3m <18.5 2 ft
QL 4 imagery 0.04 pts/m2 5 m2 ∼10m 46.3 cm2 - 139 cm2 2-15 ft
QL 5 IFSAR 0.04 pts/m2 5 m2 ∼10m 92.7 cm2 - 185 cm2 10-20 ft

 

What is a DEM?

A Digital Elevation Model (DEM) is a raster, or grid, based representation of elevation values. Each cell, or pixel, contains x and y coordinates for location as well as a z value for elevation.

 

What is the difference between Shaded Relief (dynamic color) and Shaded Relief?

The shaded relief layers are displayed with a color ramp where lower elevations are light blues (left in the ramp below) and higher elevations are (maroon to white (right in the ramp below). The Shaded Relief layer maintains the same color no matter what the zoom level. The Shaded Relief (dynamic color) layer changes color as the zoom changes so that there is always a wide color range in the view. This helps to show contrast between elevations even in a small area but can be misleading as a small hill can be colored the same as the tallest point in the state.

 

What is azimuth and altitude?

Azimuth is the angular measurement in a spherical coordinate system. Here, it is the direction (north, east, south, west) and its associated degrees in a circle, where the sun is shining from. Altitude is the vertical elevation of an object above a surface. Or here, how height on the horizon of the sun. Straight above is 90°(no shadow) and ground level is 0°. If you are an ArcMap user, check out the Illumination tab in the Data Frame properties for an interactive display of azimuth and altitude.

Wikipedia has a nice graphic and explanation.