The significance of Quality Assurance and Quality Control in modern day photogrammetry cannot be overstated. Quality should be the buzzword of the mapper who is obligated to establish professional quality control procedures.

Map Accuracy Standards as a Function of Map/Photo Scale

 

The following is a table outlining Aerial Mapping Solution’s mapping capabilities and associated map accuracies with respect to photo and map scales utilizing first-order fully analytical instrumentation. This is a general outline. The photo scale may vary depending on each individual project

Map Accuracies as a Function of Photo/Map Scale

 

 

NOTE: the specified accuracies correspond to the Federal Geodetic Data Committee spatial data  accuracy standards (ASPRS Class II Map Accuracy for large scale maps) and are valid for well-defined points.

 

 

MAP ACCURACY STANDARDS

National Map Accuracy Standards

The following information on mapping accuracies has been extracted from the Manual of Photogrammetry:
 

The accuracy of a map is determined by comparing the position and/or elevation of a feature on a map with the position and elevation of that feature as determined by field surveys. This comparison presumes that there is no discernible error in the survey.

 

The Office of Management and Budget has stated the following:
 

Horizontal Accuracy. For maps on publication scales larger than 1"=20,000', not more than 10 percent of the points tested shall be in error by more than 1/30 inch, measured on the publication scale; for maps on publication scales of 1"=20,000' or smaller, 1/50 inch; for highways, 1/40 inch. These limits of accuracy shall apply in all cases to positions of well‑defined points only. “Well‑defined” points are those that are easily visible or recoverable on the ground, such as the following: monuments or markers, such as bench marks, property boundary monuments; intersections of roads, railroads, etc.; corners of large buildings or structures (or center points of small buildings), etc. In general what is “well defined” will also be determined by what is plottable on the scale of the map within 1/100 inch. Thus while the intersection of two road or property lines meeting at right angles would come within a sensible interpretation, identification of the intersection of such lines meeting at an acute angle would obviously not be practicable within 1/100 inch. Similarly, features not identifiable upon the ground within close limits are not to be considered as test points within the limits quoted, even though their positions may be scaled closely upon the map. In this class would come timber lines, soil boundaries, etc.

 

Vertical Accuracy. As applied to contour maps on all publication scales, shall be such that not more than 10 percent of the elevations tested shall be in error more than one‑half the contour interval. In checking elevations taken from the map, the apparent vertical error may be decreased by assuming a horizontal displacement within the permissible horizontal error for a map of that scale.

 

The following is from The Reference Guide Outline (The Photogrammetry for Highways Committee, 1968):

 

Contours. Ninety (90) percent of the elevations determined from the solid‑line contours of the topographic maps shall have an accuracy with respect to true elevation of one‑half contour interval or better and the remaining ten (10) percent of such elevations shall not be in error by more than one contour interval. This accuracy shall apply only to the contours which are on each map. Thus, in each particular area where the intermediate contours have had to be omitted because of the steepness of the ground slopes and only the index contours are delineated on the maps, the accuracy stipulations apply to contour interval of the index contours. Wherever the intermediate contours are not omitted, of course, the accuracies are applicable to the contour interval specified for the topographic maps. In densely wooded areas where heavy brush or tree cover fully obscures the ground and the contours are shown as dashed lines, they shall be plotted as accurately as possible from the stereoscopic model, while making full use of spot elevations obtained during ground‑control surveys and all spot elevations measured photogrammetrically in places where the ground is visible.
 

Coordinate Grid Lines. The plotted position of each plane coordinate grid line shall not vary by more than one one‑hundredth (1/100) of an inch from true grid value on each map manuscript.
 

Horizontal Control. Each horizontal control point shall be plotted on the map manuscript within the coordinate grid in which it should lie to an accuracy of one one‑hundredth (1/100) of an inch of its true position as expressed by the plane coordinates computed for the point.
 

Planimetric Features. Ninety (90) percent of all planimetric features which are well‑defined on the photographs shall be plotted so that their position on the finished maps shall be accurate to within at least one‑fortieth (1/40) of an inch of their true coordinate position, as determined by the test surveys, and none of the features tested shall be misplaced on the finished map by more than one‑twentieth (1/20) of an inch from their true coordinate position. The true coordinate position shall be determined by making accurate measurements originating and closing on station markers of the project basic control survey, which shall have a closure accuracy conforming with the requirements for the basic control.
 

Spot Elevations. Ninety (90) percent of all spot elevations placed on the maps shall have an accuracy of at least one‑fourth (1/4) the contour interval, and the remaining ten (10) percent shall be not in error by more than one‑half (1/2) the contour interval.
 

Both versions of mapping accuracy guidelines are widely used in specifications for mapping projects.
 

Source: Combs, John E. “Planning and Executing the Photogrammetric Project.” Chapter VII of Manual of Photogrammetry, 372‑373. 4th ed. Virginia: American Society of Photogrammetry, 1980.