Using Light Detection And Ranging (LiDAR) To Find Potential Cave Areas

March 2

A new technique using remote sensing to find potential caves has recently become available on the internet. The technique is call Light Detection and Ranging, or LiDAR. LiDAR is an optical technology that uses the properties of scattered light to find the range to, or other properties of, a distant object. Like radar, which uses radio waves, LiDAR uses measurement of the time delay between when a pulse is sent to an object, and when the reflection is detected. Basically, this is the same technology that the military uses to find the range to a target, or that gets you a speeding ticket when the State Police have you in their crosshairs.

The advantage of LiDAR for cave searches is that the resolution is detailed enough that sinkholes can be easily spotted, and LiDAR can see through the forest canopy. The vertical resolution is approximately two feet, so the shallowest sinkhole may be discernable. National elevation data (NED) generally comes in three horizontal resolutions; 1arc second (30 meter), 1/3 arc second (10 meter) and 1/9 arc second (3 meter). We are interested in the 1/9 arc second data.

The Federal government is using this technology to create a very high resolution digital elevation model (DEM) for the United States using airplane overflights. This is apparently a work in progress, but data for approximately the western two thirds of Pennsylvania has been flown and is available. Unfortunately, not all of those data are available on the USGS website I describe below. Southwestern Pennsylvania is not on the server (sorry, you Loyalhanna cave searchers). West Virginia is fully available. Pennsylvania data can be purchased from the state Department of Conservation and Natural Resources (DCNR) for use in software packages that filter the data for various uses by professionals. However, there is a free source that is quite adequate for cave searches.

First, I will walk you through a few examples of how LiDAR can be used to locate potential caves. The first example below is from the area around Tytoona Cave in Sinking Valley, Blair County, Pennsylvania.

The large sinkholes at Tytoona and nearby are quite obvious in this example. Arch Spring, which is essentially a collapsed sinkhole at the edge of a hill, is also evident. Also, numerous shallow depressions, likely sinkholes, can be seen just to the west of Arch Spring in a line pointing toward Tytoona. Hopefully, these are as evident in your printed version as they are on a computer screen.

As an aside pertaining to the vertical resolution of this technology, if you look closely at the hill to the left of Tytoona Cave, you may be able to see several fences and even the pattern of row crops. These are evident on the computer screen, but may not come though well in print. Also, on a LiDAR user’s chat board one user was complaining that in Alaska artificial depressions can be seen in data collected in winter because people there empty their swimming pools in the winter.

The next example is to the southwest of Tytoona in Sinking Valley, near Fort Roberdeau. Numerous sinkholes and sinkhole-like depressions are seen across the valley. The group of sinkholes to the west are in a forested area, which has been partially outlined by a green line. Forest shows up on these LiDAR images as a subtlety stippled texture. Just to the south of these sinkholes near the edge of the figure, two streams flowing off the mountain to the west can be seen sinking.

So, as you can see this tool can be very useful at identifying areas to explore for caves. A lot of time and huffing and puffing while ridge walking can be saved by specifically targeting a spot. However, LiDAR will not will not dig open an entrance to the cave before you get there! That still requires the huffing and puffing.

Using National Map Seamless Server

LiDAR data and plots of those data are available on the USGS National Map Seamless Server at this link: http://seamless.usgs.gov.

Once you get to the National Map Seamless Server (NMSS), click on the large icon with the map of North America. This will bring you to a map overlay (a little globe may spin for a bit). You will automatically be in zoom mode (see the tools menu on the left). Move the cursor to Pennsylvania, click and hold, and move the mouse to box out the state. This will zoom you to the state. Continue to zoom in by this method until you have gotten down to the specific area you want. One thing to remember is that you are cranking a lot of data, so nothing happens quickly, even with a high speed internet connection.

Once you have zoomed to the area of interest, go to the menu to the right of the map and click on Elevation. There will be check marks on 1 arc second and on Color Shaded Relief. Uncheck these by clicking. Again, be patient. The unchecking can take some time, depending on your internet connection speed. When this is done, check the NED Shaded Relief 1/9 arc second –East button (and wait some more). The high resolution DEMs you see above will eventually come up, unless these data are not available in the area you are searching.

You can zoom in and out using the tool menu. The figures above are about at about 1:17,000 (inches). If you get much below 1:9,000 the resolution starts getting fuzzy.

If you want to pan, click and hold, move the cursor in the opposite direction from where you want to pan, and, you guessed it, wait a while.
I have not had luck using the download function for these data. If you go to the print tool a pdf file will be created (sometimes) and that can be saved. This seems to be hit and miss, though. The figures above are just screen shots, and seem to be quite adequate for saving an image. This is done by holding down Shift, and hitting the Print Screen function key at the top of your keyboard. Then go to word processing and paste the image. Consult your word processing Help to learn how to crop and expand these images.

I have used LiDAR to identify many targets in Blair County. I have checked out several, but no new entrances yet. So, let’s find those sinkholes using LiDAR, get digging, and open some new caves.

By Dave Field