This presentation reviews open source software to digitize LiDAR images of Carolina Bays
for fitting them with ellipses using the least squares method.
This is an important technique that helps to reconstruct the rims of Carolina Bays that
are only partially visible due to overlaps.
Welcome to another edition of the Carolina Bay of the Day where we study the secondary
impacts made by the glacier ice boulders that were ejected by one or more extraterrestrial
impacts on the Laurentide Ice Sheet by the Great Lakes.
The secondary impacts produced seismic vibrations that liquefied unconsolidated soil, and the
oblique impacts of glacier ice created inclined conical cavities that became shallow elliptical
basins by viscous relaxation.
In Nebraska, these basins are called Rainwater Basins, and along the Atlantic Coastal Plain
they are called Carolina Bays.
A previous presentation explained the use Google Earth to obtain a set of latitude and
longitude coordinates along the rim of a Carolina Bay.
The coordinates were then input into a program to fit an ellipse to the points by the least
squares method.
The resulting ellipse provides a robust method of finding the width and length of a Carolina
Bay that is more reliable than just taking single measurements of these dimensions with
the ruler tool from Google Earth.
I wanted to find a program to plot the points of LiDAR images that I already had on my computer
without having to go back to Google Earth.
I searched for open source plot digitizers and found a couple of potential candidates.
The advantage of open source software is that it is free and does not have pop-up ads or
the nagging requests for registration and renewal.
The first page brought up a program called plot digitizer,
and a second program called WebPlotDigitizer.
I decided to try both of these programs.
The Plot Digitizer program is coded in Java and allows the user to take a scanned image
of a plot and digitize values from the plot just by clicking the mouse on each data point.
The program is eight years old and it is not being actively maintained.
Its auto-digitizing feature is based on an image vectorization program that no longer
works on the latest operating systems.
I tried the manual digitization, and it worked as described, but it was very difficult to
calibrate the coordinates.
I then tried the WebPlotDigitizer which is a semi-automated tool that makes the process
of plotting extremely easy.
The program works with a wide variety of charts with XY coordinates, polar coordinates, and
maps with scale bars.
Automatic algorithms make it easy to extract a large number of data points.
WebPlotDigitizer was developed by Ankit Rohatgi and it is available as a web application and
desktop versions for Windows, MacOS, and Linux.
I tried the Windows version on my desktop computer with Windows 11 operating system
and the Linux version on a laptop running Ubuntu 22.04.
The operation of the WebPlotDigitizer program is very intuitive and there is a video that
explains it in detail.
I used WebPlotDigitizer on a LiDAR image with a scale bar that I had obtained from Google
Earth for an earlier project.
The program prompts you to select two points along the scale bar.
The scale bar was at the bottom of the image and I clicked and adjusted the position of
the end points.
Next, the program prompted me for the scale size measurement.
In this case the scale bar indicated 586 meters.
It was then a simple task to run the cursor along the perimeter of the Carolina Bay.
Each click created a coordinate pair.
A small square on the right of the screen magnifies the position of the cursor for more
accurate positioning.
It is possible to delete points that are misplaced and then add points at the correct location.
The data points can be displayed by clicking on the View Data button on the left of the
screen.
The panel allows adjusting the precision of the coordinates and it has options for exporting
the values to the clipboard, for downloading them as comma separated values, or to plot
them.
WebPlotDigitizer can display a graph of the data immediately using an external online
program called Plotly.
I used version 4.5 of WebPlotDigitizer, which assumed that the origin of the map was at
the top left of the image.
For this reason, the image displayed by the plotting program was a vertical reflection
of the LiDAR image.
I copied the coordinates
and pasted them into a spreadsheet.
A minor arithmetic adjustment allowed me to flip the Y coordinates of the image.
I was then able to use the least squares program to fit the ellipse for the Carolina Bay.
The dimensions of the ellipse were very similar to what I obtained with the latitude and longitude
coordinates from Google Earth.
This seemed like a good method for working with LiDAR images with scale bars.
I thought that the WebPlotDigitizer would be more useful if the origin of maps with
scale bars could be set at the bottom left of the image.
For this reason, I submitted an update request through Github which is the repository for
the program.
I was really amazed!
Within 6 hours from the time that I submitted the request on Github, Ankit Rohatgi had already
coded an option to place the origin of the coordinates at the bottom left corner of the
image.
The feature will be available in the next release version of the WebPlotDigitizer.
The response was really fast.
I am very thankful to Ankit Rohatgi for making this program publicly available.
I am sure that WebPlotDigitizer with this new option will be very useful for digitizing
geological features in images from Google Earth or any other maps with scale bars.
Thank you for joining me in the investigation of the Carolina Bays and the Younger Dryas
cataclysm.
I will continue to examine the Carolina Bays one bay at a time.
My book about the Carolina Bays is available at Amazon.
Please subscribe to my YouTube channel to be notified of future videos about the Carolina
Bays.
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