APPENDIX G — PHOTOGRAMMETRY ACCURACY ANALYSES APPENDIX G — PHOTOGRAMMETRY ACCURACY ANALYSES APPENDIX G is a spreadsheet that computes the vertical accuracy of 40 houses surveyed photogrammetrically in 2000 by the Philadelphia District of the U.S. Army Corps of Engineers (USACE). The Corps contracted with an aerial survey firm to survey photogrammetric spot heights at the adjacent grade of all house corners visible in stereo (normally 2 or 3 spot heights per building), and the Corps contracted with a land survey firm to measure vertical offsets on-site, either up or down from selected spot heights as necessary to compute the elevation of the top of bottom floor, including basements and enclosures. Dewberry's goal for its FEMA study was to estimate the vertical accuracy of this process. Ground truth surveys were performed in 2003 by Greenhorne & O'Mara, as subcontractor to Dewberry, which randomly selected 40 houses from a much larger dataset and surveyed their top of bottom floor elevations using a combination of GPS and conventional survey techniques. All surveys were performed using the NAVD 88 vertical datum. The major statistics are highlighted in green at the bottom of the spreadsheet. The average elevation error is 0.34 ft, and the dataset tested 1.19 ft vertical accuracy at the 95% confidence level. This almost exactly equaled the accuracy expected from random elevation points interpolated between 2' contours, i.e., 1.2 ft at the 95% confidence level. At the 95% confidence level, two of 40 points will have errors larger than 1.19 ft; in this case, the two addresses with top of bottom floor errors larger than 1.19 ft are highlighted in yellow on the spreadsheet. This test verifies that conventional photogrammetric standards can be applied to photogrammetric spot heights supplemented with vertical offset measurements to indirectly determine top of bottom floor elevations to traditional photogrammetric standards. For other aspects of this FEMA study, the validity of this survey method was also extended by Dewberry to the use of on-site vertical offset measurements to indirectly determine top of bottom floor elevations from other forms of aerial remote sensing, i.e., LIDAR and IFSAR, which also provide adjacent grade elevations at varying levels of accuracy. Evaluation of Alternatives in Obtaining Structural Elevation Data Dewberry APPENDIX G — ACCURACY OF PHOTOGRAMMETRIC SPOT HEIGHTS WITH MEASURED OFFSETS Susquehanna comparison of Top of Bottom Floor (TBF) surveyed elevations with LAG/HAG spot elevations from conventional photogrammetry; TBF elevations from manual LAG offset measurements 288 ID # TBF per TBF per Ft. Ft. Ft2 Street Address Structure Photo/Offset GPS Svy .. abs .2 and City 7201 440.90 440.86 0.04 0.04 0.0016 600 Island Street, Northumberland 7298 439.90 439.79 0.11 0.11 0.0121 418 Park Street, Northumberland 7280 444.20 444.00 0.20 0.20 0.0400 275 Guyer Street, Northumberland 7239 442.90 443.01 -0.11 0.11 0.0121 540 Island Street, Northumberland 7130 433.60 433.72 -0.12 0.12 0.0144 635 Park Street, Northumberland 7172 457.30 457.66 -0.36 0.36 0.1296 630 Park Street, Northumberland 17487 530.10 530.00 0.10 0.10 0.0100 25 West Union Street, Shickshinny 17513 512.80 512.76 0.04 0.04 0.0016 10 South Main Street, Shickshinny 17470 511.70 511.70 0.00 0.00 0.0000 20 West Union Street. Shickshinny 17453 530.70 530.86 -0.16 0.16 0.0256 26 West Union Street, Shickshinny 17519 518.80 518.78 0.02 0.02 0.0004 27 East Union Street, Shickshinny 83491 472.20 472.16 0.04 0.04 0.0016 290 East 9th Street, Danville 83549 471.60 471.60 0.00 0.00 0.0000 262 East 9th Street, Danville 83521 471.90 471.55 0.35 0.35 0.1225 251 East 9th Street, Danville 83543 471.00 471.00 0.00 0.00 0.0000 243 East 9th Street, Danville 83653 470.50 470.66 -0.16 0.16 0.0256 224 East 9th Street, Danville 22054 476.20 476.20 0.00 0.00 0.0000 114 South Street, Catawissa 22032 467.50 466.60 0.90 0.90 0.8100 112 South Street,Catawissa 21926 468.30 468.70 -0.40 0.40 0.1600 117 South Street, Catawissa 21886 461.20 462.80 -1.60 1.60 2.5600 101 South Street, Catawissa 21836 462.10 463.70 -1.60 1.60 2.5600 138 South 1st Street, Catawissa 21759 462.10 463.00 -0.90 0.90 0.8100 134 South 1st Street, Catawissa 21720 472.90 473.70 -0.80 0.80 0.6400 Old Feed Mill, 1st Street, Catawissa 21672 462.50 462.10 0.40 0.40 0.1600 124 1st Street, Catawissa 21481 471.40 470.23 1.17 1.17 1.3689 102 1st Street, Catawissa 19083 453.40 452.94 0.46 0.46 0.2116 319 Mahoning Street, Danville 19091 460.70 460.65 0.05 0.05 0.0025 315 Mahoning Street, Danville 19104 452.30 452.88 -0.58 0.58 0.3364 319 Mahoning Street, Danville 19115 461.50 461.80 -0.30 0.30 0.0900 307 Mahoning Street, Danville 19120 453.40 453.51 -0.11 0.11 0.0121 305 Mahoning Street, Danville 19128 453.60 453.53 0.07 0.07 0.0049 301 Mahoning Street, Danville 17398 511.10 511.12 -0.02 0.02 0.0004 69 Canal Street, Shickshinny 17411 510.50 511.00 -0.50 0.50 0.2500 67 Canal Street, Shickshinny 17474 512.00 512.00 0.00 0.00 0.0000 43 Canal Street, Shickshinny 17458 523.00 522.92 0.08 0.08 0.0064 51 Canal Street, Shickshinny 17287 517.50 516.59 0.91 0.91 0.8281 156 Canal Street, Shickshinny 21684 467.10 467.00 0.10 0.10 0.0100 208 Legion Road, Catawissa 21696 476.60 476.01 0.59 0.59 0.3481 206 Legion Road, Catawissa 21682 469.70 469.73 -0.03 0.03 0.0009 204 Legion Road, Catawissa 21676 465.80 466.11 -0.31 0.31 0.0961 202 Legion Road, Catawissa 0.34 1.19 11.6635 0.29 0.54 Avg 95th percentile MSE RMSE Accuracy equals that of random points interpolated between 2' contours, i.e., 1.2' at 95% confidence level.