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?

How does this elevation differ from other elevation data and contours on CT ECO?

How do I know what dataset I'm looking at?

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

In areas of overlap, can I change which dataset I'm looking at?

Is there a way to view only one dataset?

What is a DEM?

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

What do the numbers on the ASPECT 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?

The elevation layers are all in the Connecticut State Plane NAD 83 feet coordinate system. The elevation, or z values, are NAVD88 meters. NAVD stands for North American Vertical Datum of 1988. Read more about it here and here.

 

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 best way to use the data as here is to connect to the map 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. Specific instructions for using these tools with each Connecticut Dataset can be found on the Lidar Information page.

 

How does this elevation differ from other elevation data and contours on CT ECO?

There is an Elevation and Bathymetry map service that has been available on CT ECO for a long time. It includes a hill shade and shaded relief map as well as contours (2ft, 5ft, 10ft, 20ft, 50ft, 100ft) all based on Lidar that was collected in 2000. This map service is still available. For more information, visit the 2000 Contour Data Guide, the 2000 Shaded Relief Data Guide, the 2000 Hillshade Data Guide. To compare, the 2000 data is not as detailed as what is available now. It is also in a format that is not easily manipulated.

 

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.

 

How do I know what dataset I'm looking at?

There are many locations where datasets overlap. The footprints layer was created for this very purpose. The Elevation Viewer contains the layer called Footprints. The viewer help page includes instructions on turning on the layer in the viewer as well as using the pop-up to get the footprint information you are after.

To connect to the footprints layer in order to add it to a map, you will need to use the REST endpoint as explained here.

 

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

 

In areas of overlap, can I change which dataset I'm looking at?

Yes, if you are using ArcMap. The mosaic is dynamic and you can change the drawing order. Go here to read how. If you are not using ArcMap, then I don't know of a way to make the change. If you do, send us an email.

 

Is there a way to view only one dataset?

Yes, if you are using ArcMap. The Definition Query is the way to go. Go here to learn more. If you are not using ArcMap, then I don't know of a way to view only one dataset. If you do, send us an email.

 

What is a DEM?

A Digital Elevation Model (DEM) is a representation of elevation values over a topographic surface by a regular array of z-values. Each cell, or pixel, contains an x and y coordinate to give it location as well as a z value to give it elevation.

 

What is Slope?

Slope calculates the rate of change of elevation for each DEM cell. Slope is shown in degrees MORE See Under the Hood for more details about the slope function.

 

What do the numbers on the ASPECT map mean?

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 Explained

 

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.