LiDAR Cave Overlay, a method

LiDAR (Light Detection and Ranging), as defined by wikipedia: ” is a method for determining ranges (variable distance) by targeting an object with a laser and measuring the time for the reflected light to return to the receiver. Lidar can… be used to make digital 3-D representations of areas on the earth’s surface”. In recent years, airplanes equipped with LiDAR equipment have been utilized to create highly detailed maps of much of the earth’s surface. The increased accuracy and high frequency of detail over traditional topographic maps have numerous advantages for cavers. Cave Cartographers can increase the legibility, aesthetic appeal, and accuracy of their maps by incorporation of LiDAR in cave overlays. These maps are produced in a variety of methods, using a variety of computer programs. Below I will explore one method of producing such a map. Due to my greater familiarity of drawing software as opposed to GIS expertise the following method uses GIS to a minimal extent.

The overlay process can be broken down into four steps:

Data Reduction
Drawing the Cave Footprint
Producing a LiDAR Derived Elevation Map
Assembling the Overlay

Data Reduction (production of the lineplot)

As with the production of any cave map, the first step of the process is data reduction. This is the point at which the raw data (survey shots) from the cave are processed, usually by computer program, creating various representations of the information. Products of the reduction include two- and three-dimensional line plots of the cave system. Various software is available and employed by cavers. Some of the popular programs include Compass, Walls, and CaveWhere. Utilizing one of these programs, our objective will be to export a georeferenced lineplot representation of the cave system in a format which can be imported into GIS (geographic information system) software. Typical file types suitable for the task, which are easily exported from the popular softwares, are .shp and .kml.

If using Compass Cave Software, open the file tree, select the .DAT file, and click the “edit file node” button

From this menu, click the “Links/Fixed Stations” tab. Here survey stations from the cave may be tied to geographic locations on the earth’s surface. Using the Geo-Calculator button provides further options for defining the location of entrance stations, or other stations located with radio technology. It is important at this stage to also check that proper steps to account for magnetic declination have been taken. Input of survey dates are crucial to proper declination calculations!

Once the cave has been tied to a geographic location the lineplot can be exported in an appropriate format. With Compass, open the “Cave Viewer” to see your lineplot then click File > Export > Export 2d/3D Formats. From this menu screen the tabs can be used to navigate to the various export file type. Using the options here we need to export a .shp or .kml of the survey shot lines. Use the appropriate options for your dataset and cave (feet vs meters, datum types such as WGS 84, NAD 83, etc may vary by project). I find that a 1-pixel width export of the survey shot lines as a .kml to be useable for overlays as well as doubling its use for projects in google earth. If exporting a .shp select “survey shots” and be sure the appropriate box is checked for feet or meters.

Drawing the Cave Footprint

After exporting the lineplot, we will prepare the footprint of the cave used for overlay. This is simply a 2d representation of the plan view of the cave, with no interior detail. Cavers utilize a variety of softwares when drawing cave maps, Adobe Illustrator and Inkscape being two of the most popular. Whatever computer programs used, the basic processes of importing sketches onto the lineplot and then creating a digital representation of these sketches remains the same. The actual drawing method varies between programs and individual style. One similarity between programs is the utilization of layers which can be toggled on and off, making the editing and drawing process much simpler than creating maps on paper. By segregating our lineplot to one layer and cave walls onto another, we can toggle off all the details which are unneeded.

Selecting only the lineplot, we can copy and paste into a new document where we will build our overlay. The walls of the cave should be copy/pasted into the overlay document twice into different layers (footprint and walls). I prefer to group (ctrl+g in illustrator) the walls as one unit, and the lineplot as a unit. This makes it easy to drag and align the walls, lineplot, and what will be our footprint, without individual pieces being separated or left behind.

Next, lock and hide all layers except for “footprint”. We now want to connect the lines representing walls, into a closed shape. In illustrator this is easily accomplished by using the direct selection tool to select the endpoints of lines and join them (ctrl+j). This process should be repeated at all open sections of the perimeter of the cave (any leads, entrances, too tight, etc which are open).

Once the walls of the cave are closed into one continuous loop, we want to fill the shape rather than representing the walls as a line. In illustrator, with the loop selected, use the swap fill and stroke button on the toolbar so that the stroke is none and the fill is represented in a color specified to your personal preference. Use the opacity slider to make the shape transparent, again to personal liking (I find ~50% to work well for most maps).

If your cave is simple, with no interior columns, then your footprint layer can be locked. If there are columns, you will need to delete these lines from the footprint layer, and then modify your filled shape to represent these as voids. By drawing loops into the footprint along the walls of the columns (making the walls layer visible, the column walls can be used as a guide) we can easily accomplish this task of creating voids. Be sure that the ends of your loop overlap so there will be no voids in the passageways.

At this point lock and make all layers visible. Save your document for use later in the assembly of the final overlay.

Producing a LiDAR Derived Elevation Map

LiDAR data is typically found as a point cloud in a .las format. GIS programs can be used to analyze the information and represent it in a number of ways for various interpretations of the data. For our purposes, we are interested in a Digital Elevation Model or DEM. Frequently, .las files that have been transformed into DEM tiles can be found for download on various websites and cloud hosting services (such as the Geospatial Data Gateway). If only the raw .las file is available for the area of the cave it will need to be reduced to DEM. There are numerous tutorials available online which show this process.

The DEM can be used to produce our elevation map using ESRI’s Arcmap, the free program QGIS, or other GIS software. To my knowledge the process is nearly the same across platforms, but the example screenshots below will be from arcmap.

First, import the DEM into the map.

Next, use the hillshade tool, using the DEM as the input raster. Adjusting the azimuth and altitude can be used to highlight different features on opposite sides of hills. Sometimes the hillshade tool will be used multiple times to find the best representation of the topography.

You can see how adjusting the azimuth by 180 degrees brought out the definition of the cliff line where the entrance of our cave is located.

Next use the contour tool, processing the original DEM as the input raster, and using a contour interval that is suitable to the scale of your map and the topography of the land. Frequently I will run the contour tool twice, to produce a dataset of minor 10′ intervals and another at major 100′ graduations, in order to adjust the settings independently. This allows for adjusting color/line thickness/etc. and labeling 100′ lines quickly and easily to distinguish them from the more frequent occurrence of minor contours.

Any other datasets such as roads, streams, etc. should be imported to the map at this time.

Using the Table of Contents window, the layers we have produced can be rearranged to desired visibility, toggled on and off, and by double clicking on the title the layer properties can be opened. In the properties window the symbology tab allows for changing the physical representation of the various layers (color ramps for layers such as the DEM and hillshades are also found beneath the layer title in the table of contents). The labeling tab can be used to label contour elevations or other features. Importantly, the display tab allows for adjusting the transparency of the layers. Opacity adjustments are frequently the best way to reduce harsh lines and bright colors of entities that would distract from the overall theme of your map. Using the following settings with layers from contour at top to DEM at bottom will give you a similar base map as seen below:

100′ Contour: Line width 2pt, 10% grey color, Labeled by contour, 60% transparency

10′ Contour: Line width 1 pt, 10% grey color, not labeled, 50% transparency

Hillshade: black to white color ramp, 25% transparency

DEM: Blue Green Yellow inverted color ramp, 0% transparency

It is now time to add the lineplot to the base map we have just produced. If a .shp file was exported in the first step, the file can be added by dragging and dropping from the Catalog. If a .kml file was exported use the kml to layer tool to import the lineplot onto the map. If the location of the cave is incorrect, check that you are using the proper projection, also recheck the cave coordinates and geographic datum used in the export. Some minor adjusting of the entrance location, re-exporting, and re-importing to GIS may be necessary at this stage.

At this point the page setup (File>Page and Print Setup) should be adjusted to the proper size for the desired final product. Choosing the proper size is important for the resolution of the final map; size can vary greatly between a standalone map or an insert into a traditional cave map.

Switching from data to layout view, the map can be zoomed, and panned to ideal placement. Final adjustments of colors, line widths, etc should be made at this stage. Using the Insert tab on the file bar, wording, scales, north arrows, legends etc may be added to the map. Using the select elements tool, these objects may be moved and scaled. Double clicking most elements will open the properties tab for additional options. Once the map is fine tuned to a desirable final product and ready for overlay it needs to be exported twice. Once with the lineplot visible, and an additional identical export without the lineplot being projected. The format of export can be whatever is easiest to import into your drawing software, I find .jpg to be the easiest.

Assembling the Overlay

With the basic map components now complete the pieces can be assembled into the final overlay. For those more competent with GIS, there is likely a way to georeference and import our cave footprint directly onto our basemap. Due to my lack of competency in arcmap, I have had little luck with final assembly in GIS and have not been happy with the quality of the results. Being more proficient with the drawing software Illustrator, I find that a better-quality product can be made more quickly for me by using the following method.

In our drawing software, open the document where we drew our cave footprint in step the first step. With all of the layers unlocked and all elements of the footprint and lineplot selected, the cave footprint should be moved off of the document area while we prepare the elevation map. After moving the components lock and hide the layers. We need to now place the elevation map without the lineplot (file>place), followed by the map with the lineplot into a LiDAR layer. With both maps selected use the align tools to vertically and horizontally align the maps so they are perfectly overlapping. (The lineplot map should be on top. Right clicking and using the arrange tools can be used to move selected objects to the front or back if they are not in the correct arrangement. Temporarily making the top map semi-transparent can also assist in proper alignment)

Lock the LiDAR layer and move it to the bottom, so that the lineplot, walls, and footprint will appear on top of the basemap. Next toggle on, and unlock all the layers except LiDAR. Selecting all components of the footprint (footprint, walls, lineplot) use the scale tool (object>transform>scale) to scale up or down the footprint parts to the same size as the lineplot on the basemap. If you know the scale differences between your drawn components and the exported elevation map, a simple math problem will leave you with your scale factor. If the difference is unknown, use the scale tool to get the sizes close. You can then fine tune the scale by holding the shift key while resizing with the handles. (holding the shift key ensures a proportional adjustment in the horizontal and vertical aspects of the selected components to prevent distortion) Scale the components and align the lineplot and footprint with the lineplot on the basemap.

Once the elements are perfectly aligned, lock the layers and hide the lineplot. Unlock the LiDAR layer and delete the top map which contains the imported lineplot. The properly placed cave overlay on a LiDAR basemap is now complete. Design settings such as colors, line widths, transparencies, etc. can be finalized now. Any titles, borders and other frills should also be added in respective layers. The final product can be exported as a standalone map or can be placed as an accompanying feature to a traditional cave map.

Final Thoughts

Comparing the LiDAR map with a traditional USGS topo at the same scale, the advantages of LiDAR overlays become immediately clear. With our example cave, the bluffline where the entrance lies is not shown on the USGS map. The LiDAR product’s finer resolution is also useful in analyzing the cave’s relation to other surface features which are not represented on traditional topo maps. This detail also allows for the overlay of much smaller caves than could be produced with the low clarity basemaps available previously.

With the variety of tools available to the cave cartographer, there are an infinite number of ways to represent and analyze a single cave system. Using the method presented above as a guide, I encourage each individual to utilize their own techniques to perfect a method which works to best represent their interpretation and vision of their cave maps.