Step 2 - Looking for bays
Several methods have been used for finding and measuring the bays. The most convincing signature is found in the high resolution LiDAR digital elevation maps. However, in several situations, we see the satellite imagery being quite useful This is especially true of the Google Earth Historical Imagery, which goes back into the early and mid 1990’s where the bays show well in grey scale images. In many cases the bays have been drained and farmed upon for a hundred years or more. In those cases a remnant bay “ghost” is visible in the imagery as a darker soil surface surrounded by the thin light colored rim. In areas where the LiDAR is available, these same bay ghosts show well as depressions with raised rims. A satisfying case can be made for the identification of a bay which demonstrates both methods of detection. It then becomes a matter of personal interpretation exactly where the bay planform lies, as the dark “ghost” will typically only show the bay floor, as the rim areas do not accumulate the rich soil and moisture required.
Example 1Here is an example using historical imagery from 2/19/1993:
This image demonstrates several location hints. The dark oval with a visually smooth and level center is indicated. The raised rim of bays often directs road-building efforts around the edge of the rim, and this is demonstrated on the lower left side of the bay. It is actually quite fascinating how consistently these road “wraps” occur across the Carolinas.
A planform overlay is placed and orientated:
This particular area has LiDAR, so it is turned on to revel the DEM data:
The original overlay position seems to hold as a probable orientation. Visible in the center of the bay are the typical drainage channels, leading out to the upper left.
In this image, a very slight ghost is visible.
An overlay is placed:
The LiDAR is referenced:
.... and the overlay is adjusted to address the DEM data.
Viewing the bay without LiDAR show the adjusted overlay. Note that the raised rim does not hold water, so it will be found beyond the dark patch. Here we see that in the south east edge, the soil displays the lighter rim effect. In many instances this rim can be quite persuasive by itself.
Often the drainage channel artifacts are quite helpful. Here is a bay’s drainage showing well in the satellite imagery:
We place an initial overly test:
... and then visualize it with the LiDAR imagery:
The goal is to provide an overlay element which represents the bay’s planform and orientation well. Experience will show that many different interpretations of the same bay will still yield metrics which are very tightly constrained, typically less than +/- 5 % in values. Note that in the above imagery there is a visible difference between the LiDAR’s position on the globe vs the standard satellite imager; the LiDAR is shifted eastward by a few meters.