Evaluating the Accuracy of Digital Orthophoto Quadrangles (DOQ) in the Context of Parcel-Based GIs Joshua Greenfeld Abstract Orthophotos as a data source for compiling parcel data A crucial component in developing an effective GIs based on offer several benefits. An orthophoto offers continuous aerial digital parcel base maps is the acquisition of accumte digital coverage that includes all the existing features in the real world land base data. Accurate land base data make spatial analysis that are larger than the given pixel resolution. Orthophotos can less troublesome and enables better decision making. How- be, and usually are, georeferenced. Orthophotos are also a his- ever, acquisition of accumte spatial data from tmditional data torical record from which spatial and temporal changes can be compilation techniques could come with a hefty price tag. A derived. Advances in computer technology have made digital popular solution for the accuracy versus cost dilemma is the orthophotos more readily available. Improvements in com- use of USGS Digital Orthophoto Quadrangles (DoQ). DOQs puter processing, storage, and software have made them an provide very inexpensive continuous land covemge that could increasingly practical choice. be converted with relatively modest means and expense into Although orthophotos generally offer significant benefits, a digital parcel map. not all orthophotos are created equal. In general, their accuracy Given that are becoming a common solution for es- and quality will vary based on the data used to process them, as tablishing digital parcel coverages, it is prudent to evaluate well as on the particular production procedure. Contributing their accumcy. DOQs are considered to comply with the error factors include the characteristics and calibration of National Mapping Accumcy Standards (NMAS),but that equipment used for image capture such as the camera andlor statement does not render a constructive measure for the scanner. Additional elements affecting the rectification process accuracy of the data because of the way (or lack of) a dataset include ground control points (accuracy, amount, and distri- is certified as being in compliance with the NMAS. A better bution), the aerial triangulation process, digital elevation mod- approach for evaluating the accuracy of DWS is to follow the els (DEM)(including the elevations of features above the DEM National Standard for Spatial Data Accumcy (NSSDA) surface), and the methodlsoftware used for rectification. There- guidelines. fore, it is essential to note that orthophotos should be evaluated In this paper a DOQ was evaluated with the NSSDA for spatial accuracy and inspected for image quality before standards in order to establish the positional accuracy of the acceptance or use. data. The accuracy was found to be within 225 feet (27.6 m) The spatial accuracy evaluation of the orthophoto and its at the 95 percent confidence level. The DOQ was also evaluated suitability for a given application can be performed in a num- for its geometric, radiometric, and mosaicking accuracies. This ber of different ways. One way to do this is to overlay it onto aspect of the DOQ was found to be satisfactory. Finally, the another reference (e.g., a vector-based graphic) information appropriateness of DOQs in the context of a parcel-based GIs layer known to have higher accuracy. Differences or errors in was addressed. feature locations between the orthophoto and reference layer are observed and quantified. The results of that error quantifi- Introduction cation are used to determine the accuracy of the orthophoto. A parcel-based geographic information system (GIS)is built on Another way to test the spatial accuracy is by using the Global a data layer of reference information, such as topography, Positioning System (GPS) to determine accurate positions of buildings, road network, streams, etc., to which all other layers features that can be easily identified on the orthophoto. Com- are tied geometrically. Traditionally, this type of information paring the GPS coordinate values of these points to those mea- was developed by surveyors andlor photograrnrnetrists.While sured on the orthophoto yields the sought after spatial accuracy these methods for acquiring base map data are usually more evaluation. A guideline for such an accuracy assessment is accurate, in many cases they are prohibitively expensive for provided by the National Standard for Spatial Data Accuracy many GIs developers. Alternative, and less expensive, data (NSSDA)(FGDC, 1998). acquisition methods are digitizing existing maps and orthopho- In addition to a spatial accuracy examination, orthophotos tos. Digitizing existing maps has numerous drawbacks that should undergo an image quality review as well. Image quality include reduced spatial accuracy, incompleteness of informa- can deteriorate during image acquisition, image processing, tion, and geometrical and mathematical (coordinate system) rectification, and mosaicking. Aspects of image quality include discontinuities. Digitization is also a tedious conversion pro- cess, not necessarily inexpensive and is subject to availability of appropriate maps. The other source of data for a parcel-based GIS is a digital orthophoto. Photogrammetric Engineering & Remote Sensing Vol. 67, No. 2, February 2001, pp. 199-205. - - Department of civil and Environmental Engineering, New Jer- 0099-111210116702-199$3.00/0 sey Institute of Technology, Newark, NJ 07102 (greenfelda O 2001 American Society for Photogrammetry admin.njit.edu). and Remote Sensing PHOTOGRAMM~RICENGlNEERlNQ 8 REMOTE SENSING February 2001 199 brightness and contrast changes between images, recording of scratches and dirt as images, discontinuities at the edges of mosaicked image patches, and so forth. If some of these defects The above computed accuracy (error) value reflects all are not addressed, some local anomalies of inaccurate spatial uncertainties, including those introduced by (geodetic) con- positioning of features or systematic spatial errors may occur. trol coordinates, compilation, and final computation of ground In addition, image defects can result in incorrect interpretation coordinate values in the evaluated dataset. It is assumed that of the real-world landscape. For example, a large blob of dirt errors in the spatial data have random behavior and that sys- could be interpreted as a body of water, etc. tematic errors have been eliminated as best as possible. In this paper a Digital Orthophoto Quadrangle (DOQ)is Assuming that errors are normally distributed and indepen- evaluated using the newly endorsed NSSDA standards. The dent of each other in the Xand Y component, a factor of 2.4477 standard provides a guide for establishing the positional accu- is used to compute horizontal accuracy at the 95 percent confi- racy of spatial data. In addition, DOQs are evaluated for their dence level. When the preceding conditions apply, Accuracy,, geometric, radiometric, and mosaicking accuracies. The accu- the accuracy value according to NSSDA, is computed using the racy and the quality of DOQSare evaluated in the context of a formula parcel-based GIs. In the following section the NSSDA and the DOQ are reviewed. Following this review, an evaluation of a 2.4477 JRMSE: + RMsE$ Accuracy, = QOQ in accordance with the NSSDA guidelines is presented. 2 Findings of an image quality assessment of the DOQ are pre- sented thereafter. Finally, the appropriateness of DOQs for The NSSDA offers somewhat simplified variations for Equa- deriving digital parcel data for a GIS is also discussed. tion 4 by considering two cases. The National Standard for Spatial Data Accuracy (NSSDA) Case I: (yields exactly the same results as Equation 4) If RMsE, = RMSE, ,then The National Map Accuracy Standards (NMAS), which were established in the early 1940s, suffer from many well-docu- mented shortcomings (FGDC, 1998). Besides being incompati- RMSE, = - RMSEx = . RMSEy ble with digital mapping concepts, they have a very vague 2'4477 ' procedure for determining the accuracy of a map. The National Accuracy, = = 1.7308 . RMSE, (6) Standard for Spatial Data Accuracy (NSSDA]is a long overdue Jz successor to the NMAS. The NSSDA implements a statistical and Case 11: (yields results similar to Equation 4) testing methodology for estimating the positional accuracy of If RMSE, Z RMSE,, (the ratio between the largest RMSE and points on maps and in digital geospatial data, with respect to the smallest one is between 0.6 and LO), then the circular stan- georeferenced ground positions of higher accuracy. The NSSDA dard error (at 39.35 percent confidence) may be approximated applies to georeferenced maps and digital geospatial data, in as 0.5 X (MSE, + MSE,) (Greenwalt and Schultz, 1968) and the either raster, point, or vector format (FGDC, 1998). computed accuracy is Unlike the NUS, which sets a compliance accuracy value, the NssD.4 does not define threshold accuracy values. Rather, agencies are encouraged to establish accuracy thresholds for Accuracy, their products based on application needs and contracting requirements. It is left to users to identify acceptable accuracies for their applications. Data and map producers should deter- mine what accuracy exists or is achievable for their data and Accuracy Test Guidelines record it on an explicit reporting form. According to the Spatial Data Transfer Standard (SDTS],accu- The NSSDA uses the root-mean-square error (RMSE) to esti- racy testing should be performed using an independent source mate