Monday, March 30, 2009
SELC calls Jordan Lake boundary 'conservative'
Citing conflicting data, the Southern Environmental Law Center, a Charlottesville, Va.-based advocacy group, has called on the Durham County Board of Commissioners to reject a developer-funded survey of Jordan Lake, and commission an independent survey of the reservoir. The group based its recommendation on data collected in 2007 for the N.C. Floodplain Mapping Program that show Jordan Lake extending beyond a margin set in the 1970s by U.S. Geological Survey maps.
The controversial developer-funded survey, by contrast, shows Jordan Lake contracting in the opposite direction, thus removing property owned at the time by private developer Neal Hunter from a protected area surrounding the reservoir. In letters to state and county officials, Hunter's associates have argued the survey--which Hunter commissioned in 2005--represents "better information" than the 1970s maps, and used the difference in mapping technologies as justification for bypassing a public hearing process required by state law to implement zoning map changes.
The 2007 data SELC presents was collected via a technology known as Light Detection and Ranging (LiDAR), used by NASA and the National Oceanic and Atmospheric Administration to analyze topography. In a March 20 letter (PDF), SELC Staff Attorney Kay Bond notes that the LiDAR data "cannot substitute for an on-the-ground-survey" such as Hunter's, but "can give an accurate enough approximation of actual conditions to indicate whether the conclusions reached by a survey are generally valid."
Bond implies that Hunter's survey--which only analyzed the portion of Jordan Lake affecting his property--may not be completely accurate:
Based on these maps, it appears that the original Jordan Lake boundary recognized by USGS is a conservative one. These maps call into question the surveys submitted by the City of Durham in support of its request to adjust the watershed boundary for Jordan Lake.
View a PDF of SELC's maps, which include Neal Hunter's 2005 survey, and the original U.S.G.S. survey, here.
Comments (5)
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[...] the data than was actually given. Like many powerful tools, GIS will have to be used with care. Differences in analysis and interpretation can lead to many disputes that need not arise. However, when used well, surprising and good [...]
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Another, perhaps more important, technical problem with the Hunter map is the short length of the survey line. The line formed by the intersection of the lake surface and the ground is geometrically very complex - a scribbled zig-zag of a line. Mathematical smoothing is required to identify the position of a trendline, and it is this smoothed line that is used to define the critical watershed boundary. A short survey only tells you the position and trend of a single zig, not the average for that side of the lake. A much longer survey line - like the proposed independent survey of the entire lake - is needed to confidently identify the position and trend of the smoothed boundary line.
The SELC obtained a LIDAR analysis at 216 feet (meaning ground at 216 feet above sea level) and one at a much more generous 215 feet (which would define the lake at even lower ground). Even the 215 map shows the lake extending beyond the CURRENT maps, contradicting Hunter's results. Note that the SELC says that the lidar measurements are accurate to within 10 inches. The map at 215 feet, therefore, allows a full foot of error -- essentially eliminating any error with the 216 foot map. This is undeniable evidence that a new, truly independent survey could very well show very different lake boundaries...
Wasn't the Hunter survey performed during the DROUGHT-OF RECORD???????? I seem to remember several notes to that effect in the botanical survey that I read. Liz Pullman 31 March 2009
The LIDAR map contains thousands of data points. Hunter's survey has 16, with a 600-foot gap between two of them. As they say on TV, "questions have been raised."
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