ROCKY FORK TRACT: VIEWSHED ANALYSIS REPORT

Transcription

1 ROCKY FORK TRACT: VIEWSHED ANALYSIS REPORT Prepared for: The Conservation Fund Prepared by: A Carroll GIS 3711 Skylark Trail Chattanoga, TN 37416

2 INTRODUCTION This report documents methods and results for work completed on a viewshed analysis project for the Rocky Fork tract located in Greene and Unicoi Counties, Tennessee (Figure 1). Viewshed models are used to demonstrate visibility between fixed observation points and targets designated within digital elevation models in a Geographic Information System (GIS) environment. This particular analysis project is designed to provide The Conservation Fund and associated project partners with the information required to calculate areas visible in the Rocky Fork tract, as viewed from the Interstate 26 (I-26) transportation corridor. Area calculations and results are further defined within the context of current proposed ownership scenarios for the Rocky Fork tract. The Conservation Fund provided all materials and coordination necessary to identify specific I-26 observation points of interest and ownership scenarios. Rocky Fork Tract Viewshed Modeling Methods Digital Elevation Model (DEM) Collection The source elevation data for the Rocky Fork Tract and surrounding areas was created using downloaded USGS Digital Elevation Model (DEM) data files of 1:24,000 scale (x,y resolution: 10 meter, z resolution: 0.1 meter) in SDTS format. Initial extraction of these files into a GIS-ready format (.dem file types) was performed using SDTSEDEM.EXE conversion utility software. All DEM files have horizontal datum based on North American Datum of 27(NAD27) with a Transverse Mercator (Zone 17N) projection. Data files had varying small contour unit intervals with either 20 or 40 foot intervals. DEM Name SDTS Number Contour Interval (ft.) Greystone Bald Creek Chestoa Erwin Flag Pond Huntdale Iron Mountain Gap Unicoi Sams Gap Chuckey Telford Raster Creation and Mosaic The extracted DEM data files were converted to raster format by using the ArcToolbox DEM to Raster conversion tool. The rasterized files were mosaiced together to create a seamless coverage of the study area. The mosaic was performed 2

3 using the Mosaic To New Raster tool in ArcToolbox; the overlapping elevation model areas were calculated using the mean values of the rasters. Hillshade and Viewshed Analysis Hillshade and viewshed analyses were performed using the Spatial Analyst extension for ArcGIS Desktop 9.2. Hillshades were generated based on the mosaiced DEM raster files using the Hillshade tool. Hillshades would be employed in final cartographic designs, serving as background layers useful for depicting terrain characteristics. Following hillshade production, viewshed analyses were performed. ESRI describes the capabilities of the viewshed tool in the Spatial Analyst extension as Viewshed identifies the cells in an input raster that can be seen from one or more observation points or lines. Each cell in the output raster receives a value that indicates how many observer points can be seen from each location. If you have only one observer point, each cell that can see that observer point is given a value of one. All cells that cannot see the observer point are given a value of zero. The observer points feature class can contain points or lines. The nodes and vertices of lines will be used as observation points. 1 Viewshed analyses required the selection of multiple observation points. These model input points were identified by The Conservation Fund through coordination with the Tennessee Department of Transportation. Selected observation points of interest included: 1) Eastbound I-26 scenic overlook located north of the Tennessee North Carolina state line 2) Westbound I-26 overlook located south of the Rocky Fork tract 3) Proposed site of new TDOT welcome center at Clear Creek exit off I-26 4) Route of I-26 south of Erwin, TN to the Tennessee North Carolina state line Field data collection of observation points was performed to produce highly accurate points for analyses. Points were collected with a Trimble GeoXH handheld GPS device and hurricane antenna. Real-time differential GPS correction using the Wide Area Augmentation System (WAAS) was enabled during field data collection 2. This correction measure and a documented percent dilution of precision (PDOP) ratio < 4 for satellite reception generated geographic points with estimated sub-meter horizontal accuracy. Once added to GIS software, field data points where categorized based upon location at selected 3 fixed observation sites (points of interest 1-3 from above). Categorized points were saved as an ESRI feature class and added to the viewshed modeling environment. Viewshed observation points along the I-26 corridor were created by generating a series of points based upon a 100 meter interval along the interstate centerline file. The interstate centerline file was developed by the Tennessee Department of Transportation and published in The selected span of I-26 centerline employed in model analyses 1 ESRI, 2007., ArcGIS Dekstop Help. ArcGIS Desktop Trimble GeoXH Specification Sheet: / D_GeoXH_DS_0607_lr.pdf 3

4 was determined through a process of prescreening single observation points. Viewshed outputs from single points on the road centerline were compared to the Rocky Fork tract to detect the approximate starting point of tract visibility from the interstate corridor. Once this starting point was determined, a 100 meter interval starting point was created approximately 1000 meters north to allow for possible errors in the selection of single points from the pre-screening process. The interval continued south along the full extent of I-26 to the Tennessee North Carolina state line, generating 161 observation points. Four individual viewshed models were calculated based on the selected observation point categories listed above. All four models were combined into a cumulative viewshed model using the map algebra function in the Spatial Analyst extension. The resulting combined model depicted all visible and non-visible cells based on a total of 264 observation points (161 from I-26 & 103 from the welcome center, and east & westbound scenic overlooks). The combined model was clipped to the mask of the Rocky Fork tract. Since the elevation model was based on a 10 meter x 10 meter cell size, each model cell equaled 100 square meters. The area of visible vs. non-visible within the tract was calculated by multiplying the total cell count by 100. The resulting number in square meters was converted to acres for use in the planning process. Combined viewshed results were further clipped based upon 4 phase or priority areas comprising the larger tract. Acreages for each of the smaller focus areas were calculated using the same method of cell count x 100 square meters. Results: Tract or Area Number Model Cells Visible Acres Percent of Tract or Area Priority Area 1 2, % Priority Area % Priority Area 3 39, % Priority Area 4 50,210 1, % Total Tract 92,938 2, % (See Figures 2 & 3) Discussion Viewshed operations produced coherent models based upon the observation points defined during model preparation. However, these models and calculated areas should be used for planning purposes only. Several data properties prevent final products and calculations from adhering to survey grade quality. The DEMs used in model development maintain a horizontal accuracy of ~ 10 meters. This same raster resolution was translated to viewshed models; thus making possible horizontal error too great for survey grade analysis. DEMs employed in analysis were based on elevation of ground surface. First return features such as tree canopy and structures were not calculated in this model 4

5 approach. The use of these features would have allowed ridgelines and structures to appear from lower elevation observation points. The use of ground surface in this case was a more conservative approach. Additional offsets and viewing ranges were considered to account for surface features; however, model defaults were selected for final analyses due to time and data acquisition constraints and adherence to conservative calculations. Viewsheds were deemed to be accurate and reliable through analysis in 3D visualization software and cross-reference to field documentation at observation sites (Figure 4). All five models and acreage calculations generated reasonable estimates of viewsheds and area. 5

6 Figure 1: Location of Rocky Fork Tract 6

7 Figure 2: Viewshed model results 1:40,000 scale 7

8 Figure 3: Viewshed model results 1:75,000 scale 8

9 Figure 4: Comparison of viewshed model performed in 3D software environment and with field documentation. 9

10 Image 1: Photo of proposed welcome center area with view of Rocky Fork tract in background. 10

11 Image 2: View from eastbound scenic overlook. 11