Light Detection and Ranging (LiDAR) Elevation Data

Bryan Pittman, Sr. GIS Specialist, WSB

What is LiDAR?

LiDAR, which stands for Light Detection and Ranging, is a combination of “light” and “radar.” It’s a remote sensing technology that uses lasers to detect and measure features on the surface of the Earth. Due to its high accuracy, LiDAR has become the de facto standard for creating elevation surfaces and measuring heights of features above the ground such as trees or buildings.

LiDAR in action

Minnesota completed a statewide LiDAR gathering project funded by the Clean Water, Land and Legacy Amendment, and spearheaded by the Minnesota Department of Natural Resources and the Minnesota Geospatial Information Office. The six-year project resulted in a seamless, high-resolution digital elevation map of the entire State of Minnesota. This data is completely free to download and offers a vertical accuracy of six inches. This project has enabled the flow of accurate topographic information between all organizations and the general public.

LiDAR deliverables

The deliverables of the project came in different formats. The simplest and most frequently used format is two-foot contours that were generated statewide. There is also a high-resolution Digital Elevation Model (DEM) that can be acquired as county tiles. The user can generate contours at varying intervals in this format, such as one-foot or even six-inch. Both the contours and the DEM use bare earth returns, meaning you only get surface elevation.

A third format is the raw LiDAR data, which is dense collection of points, or a point cloud. If you imagine the laser from an airplane hitting the surface, it’s the information at that contact point that is reflected back to sensors on the plane. The density of those points depends on the exact collection methods, but typically there will be 2 million points per square mile, or approximately 20,000 points for a typical city block. The point cloud gives access to all the returns and not just the bare earth returns; therefore, we can gather information about the heights of trees, buildings, water towers, etc. The point cloud is so dense that it is even possible to extract overhead power lines from the data. These multiple returns allow the data to be used for many different 3D analyses and visualizations. Certain 3D software packages allow the user to take the point cloud and turn it on its side, creating a vertical profile with accurate object heights and ground elevations.

LiDAR uses

There are many uses for LiDAR data beyond viewing ground elevation or object heights. Any kind of hydrologic flow analysis can benefit from the use of this data. Erosion analysis can be done by using slope estimates from LiDAR to compute the amount of erosion in certain areas, and that in turn can be used to calculate sediment accumulations. LiDAR has also been used for flood modeling, urban planning, oil and gas exploration, and coastline management. With the wide availability of free and highly accurate topographic data, many are reaping the benefits of LiDAR data and finding that its uses are far-reaching across many disciplines.

Using NDVI to Generate Impervious Surfaces for Large Areas

By Bryan Pittman
Feb. 6, 2015

Calculating the area or percentage of impervious surfaces for a given spatial extent helps determine curve numbers, runoff rates, and pollutant loadings. Overlaying an impervious surface layer with drainage areas for a city can determine impervious percentage per drainage catchment. The issue is getting an impervious surface for a large enough area, for example a city or a Watershed Management Organization (WMO). Digitizing of an aerial can create impervious surfaces for small areas but is too time consuming on a large scale.

Fortunately, with the current availability of high-resolution Color-Infrared (CIR) aerial photography, there is a workaround that is far less time consuming. Since the reflectance of vegetation peaks in the near infrared, vegetation yields high returns on CIR photography. This can be used to generate a Normalized Difference Vegetation Index (NDVI). The NDVI is a ratio from the returns of near infrared and visible light, telling us how “alive” something is. A high NDVI ratio signifies healthy, growing, green vegetation, where a low NDVI ratio signifies something not living, say pavement or rooftops.

The typical NDVI value ranges from -1.0 to +1.0 (GIS software calculates a value from 0 to 200). In that range, there will be a cut-off point that separates vegetation from non-vegetation. The value is typically just above 0.0, but varies based on the CIR aerial photography being used. Classifying the NDVI surface into two groups from that cut-off point gives a result that shows the area of vegetation and non-vegetation. Since there is a very high correlation between areas of non-vegetation and impervious surfaces, this result shows what is impervious and what is pervious.

This method yields results that are about 90 percent accurate. First, the assumption is made that vegetation equals pervious surfaces, which is not always the case. A large area of open dirt is a good example. It is still pervious but shows up as impervious because it is not vegetation. Another issue is shadows cast by trees, houses, and other structures. Since shadows are blocking out the light return (both visible and near infrared), any shadow is interpreted as non-living and thus impervious, even though it may be a pervious surface.

Even with these minor disadvantages, the time saved is enormous. Instead of taking weeks to digitize all the impervious area within a city, this analysis can be completed in under an hour. It is necessary to perform quality control on the data and clean up any of the issues described above by reclassifying something as pervious to impervious or vice versa.

 

Tracking Santa: An ArcGIS Online case

By John Mackiewicz
February 6, 2015

ArcGIS Online connects maps, apps, data and people so you can make smarter, faster decisions. It gives everyone – both inside and outside organizations – the ability to discover, use, make and share maps from any device at any time. At its core, ArcGIS Online is a hosted cloud software as a service (SaaS) platform. Everything you need to create your own web maps and apps is available on ArcGIS Online. You can create maps from Microsoft Excel or upload your data from ArcMap to share your map and collect data in the field on your tablet or phone.

ArcGIS Online supports many users collecting data in the field at one time. This presents a problem for large workforces, as you may need to track where your collectors go when working in the field. Using Esri’s Collector for ArcGIS app, you can have it periodically report the location of data collectors back to a tracking layer on ArcGIS Online by publishing a tracking layer on ArcGIS Online and adding it to an Web Map with tracking enabled. When this Web Map is accessed within the collector app, the collector app sends its GPS location back to the tracking layer hosted on ArcGIS Online at a predefined interval.

At WSB, we view ArcGIS Online as a technology that:

  • Can quickly be deployed for multiple uses
  • Is flexible enough to handle diverse workflows without requiring any programming
  • Has untapped potential for public outreach

Below is one of our favorite examples of how we used ArcGIS Online to help a client deliver immediate value to both the organization and the public.

Tracking Santa

For more than 25 years, firefighters in the City of St. Anthony, Minnesota, have helped Santa by collecting gifts for those in need. Santa rides in fire trucks throughout the city collecting gift donations from residents. In 2014, the city wanted to allow residents to track Santa’s location along his route.

The City of St. Anthony decided to utilize ArcGIS Online to track Santa, thanks to all the app’s capabilities.

Here’s how we did it:

  1. A tracking layer was published on ArcGIS Online.
  2. The tracking layer was added to a Web Map configured with the city’s custom Esri base maps with tracking enabled.
  3. The city deployed an iPad with the Esri Collector for ArcGIS app to ride along with Santa with the Web Map open on the fire truck.
  4. A custom web app was built using our DataLink platform to show Santa’s most recent location.

As the fire truck drove along its route, the collector app was configured to report the truck’s location every 30 seconds back to ArcGIS Online. Residents used DataLink to view Santa’s current location in relation to their house so they knew when Santa was arriving.

Tracking Santa’s location is certainly a unique use of ArcGIS Online, but it shows how extensible the ArcGIS Online platform is. With just a few clicks, you can begin to track real-time locations of users who are using the collector app.