National Map Accuracy Standards Research Articles

Finite Mixture Models in the Evaluation of Positional Accuracy of Geospatial Data

Digital elevation models (DEMs) are highly relevant geospatial products, and their positional accuracy has demonstrated influence on elevation derivatives (e.g., slope, aspect, curvature, etc.) and GIS results (e.g., drainage network and watershed delineation, etc.). The accuracy assessment of the DEMs is usually based on analyzing the altimetric component by means of positional accuracy assessment methods that are based on the use of a normal distribution for error modeling but, unfortunately, the observed distribution of the altimetric errors is not always normal. This paper proposes the application of a finite mixture model (FMM) to model altimetric errors. The way to adjust the FMM is provided. Moreover, the behavior under sampling is analyzed when applying different positional accuracy assessment standards such as National Map Accuracy Standards (NMAS), Engineering Map Accuracy Standard (EMAS) and National Standard for Spatial Data Accuracy (NSSDA) under the consideration of the FMM and the traditional approach-based one-single normal distribution model (1NDM). For the NMAS, the FMM performs statistically much better than the 1NDM when considering all the tolerance values and sample sizes. For the EMAS, the type I error level is around 3.5 times higher in the case of the 1NDM than in the case of the FMM. In the case of the NSSDA, as it has been applied in this research (simple comparison of values, not hypothesis testing), there is no great difference in behavior. The conclusions are clear; the FMM offers results that are always more consistent with the real distribution of errors, and with the supposed statistical behavior of the positional accuracy assessment standard when based on hypothesis testing.

Quantifying local flow direction uncertainty

Absolute elevation error in digital elevation models (DEMs) can be within acceptable National Map Accuracy standards, but still have dramatic impacts on field-level estimates of surface water flow direction, particularly in level regions. We introduce and evaluate a new method for quantifying uncertainty in flow direction rasters derived from DEMs. The method utilizes flow direction values derived from finer resolution digital elevation data to estimate uncertainty, on a cell-by-cell basis, in flow directions derived from coarser digital elevation data. The result is a quantification and spatial distribution of flow direction uncertainty at both local and regional scales. We present an implementation of the method using a 10-m DEM and a reference 1-m lidar DEM. The method contributes to scientific understanding of DEM uncertainty propagation and modeling and can inform hydrological analyses in engineering, agriculture, and other disciplines that rely on simulations of surface water flow.

Assessment of cartographic potential of EgyptSat-1 satellite image (case study in flat areas)

Egypt has launched its first Earth remote sensing satellite EgyptSat-1 in 2007. A series of evaluations of the usefulness and limitations of EgyptSat-1 images for different applications are required. This research is concerned with the investigation of the geometric and cartographic potential of EgyptSat-1 image in order to know to what scale EgyptSat-1 image is suitable for map production and map updating. The geometrical accuracy of rectification of EgyptSat-1 based on non-parametric rectification (polynomial approach and rubber sheeting) has been evaluated. Four methods of rectification have been compared: different orders of 2D polynomials (first-order polynomial, second-order polynomial, third-order polynomial) and rubber sheeting method. Different sets of ground control points (observed with differential global positioning system) have been tested. Several experiments were performed to rectify EgyptSat-1 image using the four rectification methods. For each method, different numbers of ground control points (GCPs) were used starting at 15 points, and then the number was reduced each time till the minimum number of GCPs for each method is reached and with three different GCP distributions. Various issues such as the solutions, feasibility, accuracy, and requirements for control information are addressed. The results show that rubber sheeting followed by third-order polynomial then second-order polynomial then first-order polynomial gives the best root mean square error. Actually, a serious mapping should be based on a satisfying by the theory justified mathematical model so that rubber sheeting should not be used for a serious mapping even it shows smaller discrepancies, but for the flat test area, an approximate 2D transformation can be accepted. Also although third-order polynomial is accurate in the proximity of GCPs and shows smaller discrepancies compared to the first- and the second-order polynomials, they can lead to significant errors for regions outside the GCP range and should not be used for a serious mapping. One can be considered the best resulted rectified image that obtained from second-order polynomial followed by the first-order polynomial. It was found in our study area that the minimum number of control points for each method gives accuracy better than a pixel. The first-order polynomial and rubber sheeting are good from the economical cost that they use minimum number of control points (three). It was found that the planimetric accuracy of rectified EgyptSat image met the 1:25,000 mapping standard (National Map Accuracy Standards). Also from the assessment of the information content of the EgyptSat image, it was found that it is consistent with the map scale of 1:25,000. In conclusion by analysis of the results, it is obvious that EgyptSat images are able to produce and update map of scale 1:25,000.

SSTL UK-DMC SLIM-6 Data Quality Assessment

Satellite data from the Surrey Satellite Technology Limited (SSTL) United Kingdom (UK) Disaster Monitoring Constellation (DMC) were assessed for geometric and radiometric quality. The UK-DMC Surrey Linear Imager 6 (SLIM-6) sensor has a 32-m spatial resolution and a ground swath width of 640 km. The UK-DMC SLIM-6 design consists of a three-band imager with green, red, and near-infrared bands that are set to similar bandpass as Landsat bands 2, 3, and 4. The UK-DMC data consisted of imagery registered to Landsat orthorectified imagery produced from the GeoCover program. Relief displacements within the UK-DMC SLIM-6 imagery were accounted for by using global 1-km digital elevation models available through the Global Land One-km Base Elevation (GLOBE) Project. Positional accuracy and relative band-to-band accuracy were measured. Positional accuracy of the UK-DMC SLIM-6 imagery was assessed by measuring the imagery against digital orthophoto quadrangles (DOQs), which are designed to meet national map accuracy standards at 1 : 24 000 scales; this corresponds to a horizontal root-mean-square accuracy of about 6 m. The UK-DMC SLIM-6 images were typically registered to within 1.0-1.5 pixels to the DOQ mosaic images. Several radiometric artifacts like striping, coherent noise, and flat detector were discovered and studied. Indications are that the SSTL UK-DMC SLIM-6 data have few artifacts and calibration challenges, and these can be adjusted or corrected via calibration and processing algorithms. The cross-calibration of the UK-DMC SLIM-6 and Landsat 7 Enhanced Thematic Mapper Plus was performed using image statistics derived from large common areas observed by the two sensors.

Vertical accuracy of the SRTM DTED level 1 of Crete

A reference digital elevation model (DEM), produced from contour lines digitization, from topographic maps at scale 1:250.000 is used in order to assess the vertical accuracy of the SRTM DTED level 1 in Crete Island in Southern Greece. The error image interpretation revealed three types of systematic errors: (a) stripping, (b) large voids and (c) those errors resulted from the mis-registration of the Shuttle Radar Topography Mission (SRTM) imagery to the local datum. Terrain was segmented to plane regions and sloping regions. Sloping regions were segmented to aspect regions (aspect being standardized to the eight geographic directions defined in a raster/grid image). Error statistics was computed for the study area as well as the individual terrain classes. Vertical accuracy was found to be terrain class dependent. Sloping regions present greater mean error than the plane ones. Statistical tests verified that the difference in mean error between aspect regions that slope in opposite geographic directions is statistically significant. The greater mean error is observed for SW, W and NW aspect regions. The additional finishing steps applied to the SRTM dataset were not sufficient enough for the systematic errors and the terrain class dependency of the error to be corrected. The observed root-mean-square error (RMSE) for the SRTM DTED-1 of Crete do not fulfil the 16m RMSE specification for the SRTM mission while the USA national map accuracy standards for the scale 1:250.000 are satisfied.

Estimation of Error and Compliance in Surveys by Kriging

This paper demonstrates how a geostatistical method called kriging can be used to establish limits on the minimum allowable contour interval (MACI) that complies with National Map Accuracy Standards (NMAS). Kriging works with the same survey point file as ordinarily required by a digital terrain model in a conventional mapping assignment, but extracts more information from the data set. The method is not appropriate for surveys of man-made features, but it is well suited for large surveys of predeveloped parcels and GIS base mapping. Because field surveys are nonrandom samples by design, kriging is unable to calculate a precise compliance contour. However, a suitably narrow interval for the MACI can be determined. Although kriging is not as definitive as a second independent survey, it can offer an effective, expedient, and inexpensive screening tool for assessing NMAS compliance. A detailed example demonstrates application of the proposed kriging method.

Precision in geodetic correction of TM data as a function of the number, spatial distribution,and success in matching of control points: a simulation

The sources of error affecting geodetic correction of TM data have been enumerated in NASA specifications and cited in the research literature. A useful typology of these errors is the geometric errors remaining after applying systematic correction data and those errors arising from the location, generation, and application of ground control points. The relative magnitudes of the effects of these errors are studied via a simulation of the geodetic rectification process which is constructed such that operationalized (quantified) forms of errors are the simulation parameters. The kernel of this simulation is a rectification procedure called GED (geodetic error determination) which uses control point distortion data to adjust the orbit/attitude data, which in turn is used to adjust transformations between the satellite focal plane and the ground. Errors associated with base map digitization and control point generation produce the greatest impact on rectification. In the absence of such errors few control points are needed to geodetically correct the data close to National Map Accuracy Standards for 1:24,000 series maps. Evenness in the distribution of control points is not critical.

Cartographic Accuracy of LANDSAT-4 MSS And TM Image Data

Investigations of the cartographic quality of Landsat-4 MSS and TM image data in CCT-p formats have produced rectification accuracies (rmsexy values) of ± 2/3 to ± 1 data pixel for both whole and subscene areas using polynomials of the first through third degree. In order to achieve these accuracies with MSS data, 15 or more Ground Control Points (GCP's) are required, whereas with the TM data sets as few as 5-10 GCPs will suffice. Factors which limit the cartographic rectification accuracies of the Landsat-4 data include: 1) spatial resolution of the data; 2) map and digitizing errors; and 3) terrain relief. Of these factors, data resolution is the most significant, limiting the location of GCP's to about ± 0.5 pixel. Horizontal displacements due to terrain relief can be minimized by selecting GCP's at or near midrange elevations. Overall, the representative rmsexy values of ±25 and ±55 m for TM and MSS data sets are within U.S. National Map Accuracy Standards for cartographic products of 1:100 000 and 1:200 000 scale, respectively.

Proposed map accuracy standards for a multipurpose cadastre

This paper reports studies made by two National Research Council Committees, the Committee on Geodesy and the Committee on Integrated Land Data Mapping. In their two reports entitled, respectively, “Procedures and Standards for a Multipurpose Cadastre” and “Modernization of the Public Land Survey System”, they made recommendations as to base map scale, appropriate contour interval, and cadastral survey standards. This paper combines this information with the U.S. National Map Accuracy Standards to develop Multipurpose Cadastre Map Accuracy Standards which are relative to the surrounding survey control used in the cadastre development.

Mapping Massachusetts Wetlands

Government mapping programs have often been cost ineffective because of the repetition of incompatible and redundant mapping efforts at the conclusion of which the resultant maps are often not available for reuse or are not applicable to other purposes. These conclusions are drawn from a critical review of a mapping program which has evolved into the development of a wetlands restriction program for the Commonwealth of Massachusetts, mandated by statute and administered by a state agency. The subject mapping program has evolved from a process of annotation directly on contact prints of aerial photographs to the production of ground-controlled orthophoto maps prepared to National Map Accuracy Standards, but the purposes of the programs have not yet been achieved. The program is at a standstill while the subject of litigation in the courts of Massachusetts and of thorough review by the Massachusetts Attorney General.

Geometric and cartographic accuracy of ERTS-1 imagery

The potential applications of the RBV and MSS images in cartographic mapping is limited by resolution rather than by geometric fidelity. A bulk RBV image from ERTS-1 should have sufficient geometric fidelity to meet the National Map Accuracy Standards for mapping at 1:500,000 scale. With four or more control points for distortion correction, a bulk MSS frame could also be processed to meet the NMAS requirement of 1:500,000-scale mapping. In digital processing, a relative positioning accuracy of plus minus 55m should be attainable for both RBV and MSS images. High-order polynomials were found to be very effective in modeling the total geometric distortions in both the RBV and MSS systems.