Design

Aerial Surveying

Formerly Federal-Aid Policy Guide G 6012.5 Chapter 6,
December 9, 1991, Transmittal 1
See Order 1321.1C FHWA Directives Management

1. Purpose
2. Definitions
3. Specifications
4. Aerial Surveying Procedure

1. Purpose. To present as a guide to transportation engineering organizations the potential uses of aerial surveys and the recommended sequence of operations to achieve maximum efficiency and effectiveness from aerial surveys.
2. Definitions
   1. Aerial Surveys for Transportation Engineering -the taking and use of aerial photography and other imagery for accomplishing the engineering and associated work.
   2. Photogrammetry - the quantitative aspect of aerial surveys, based on the science or art of obtaining reliable measurements by means of photographs, which produces dimensional data for mapping, cadastral purposes, design, and computation of earthwork and other construction quantities.
   3. Interpretation - the result of stereoscopic examination of aerial photography augmented by other imagery to obtain qualitative information about the natural terrain and cultural features which may influence the location of a transportation facility.
   4. Imagery - includes the aerial data collected by systems other than cameras (e.g., thermal infrared, radar). The data collected by cameras is referred to as photography.
3. Specifications. Aerial surveys may be performed by personnel of the organization responsible for designing the facility, or the services of photogrammetric engineering consultants may be utilized. In either case, the adequacy of all aerial surveying should be judged by the standards in the publication Reference Guide Outline -- Specifications for Aerial Surveying and Mapping by Photogrammetric Methods for Highways, U.S. Government Printing Office, 1968. The provisions of these specifications should also be followed when negotiating contracts with photogrammetric consultants.
4. AERIAL SURVEYING PROCEDURE. Utilization of aerial surveys does not alter the long established sequence of transportation engineering work. Properly used, aerial surveys can result in better engineering, usually at a lower cost. The most efficient and effective procedures for using aerial surveys are within the following fundamental sequence of engineering stages:
   1. Planning is the stage in which needs are determined and termini, standards, and priorities are established. Aerial surveys can be a useful tool to the planner.
      1. Satellite imagery and very high flight height aerial photography reveal land use patterns useful in regional planning.
      2. Periodic small scale photography of urban areas shows urban and suburban growth patterns.
      3. Aerial photography is useful in traffic engineering, both to develop models for analyses and to study specific problems.
      4. Periodic large scale photography can reveal parking patterns.
      5. Large scale photography can be used to estimate distribution of population and of activities which generate traffic.
   2. Reconnaissance Survey of Area is the stage in which all feasible route alternatives between the termini are determined. Aerial surveys provide the most complete and easiest way to perform this work.
      1. Existing small scale photography is obtained or, where culture changes rapidly, new small scale photography is taken of a large area which includes the termini. Generally, the length of the area is at least 1.1 times, and the width is approximately 0.6 times the distance between the terminal or intermediate control points. The best available maps of the area are used to supplement the photography.
      2. The photography is interpreted to determine the topography, geology, land use, soils, ecology, and other factors influencing route location.
      3. Each feasible route alternative is determined and is then drawn, in stereoscopic correspondence, on the photographs. It may also be depicted on an existing topographic map, or the photographs may be assembled into a mosaic which serves as a map.
      4. Where gradient governs location, available large scale topographic maps may be used, or the route may be placed in its best lo cation directly on the photographs by use of parallax measurements on stereoscopic pairs of the photographs.
   3. Reconnaissance Survey of Route Alternatives is the stage in which each feasible route alternative is evaluated and one of them is chosen as the corridor within which the transportation facility will be located and designed. Each feasible route is evaluated for impact on the environment, service to the public, and economic advantage. Aerial surveys continue to be the best tool for this work, using, in most cases, the photography secured for the previous stage.
      1. Impact on the environment is assessed by interpretation of the photography or imagery.
      2. Where approximate construction costs are necessary to help in deciding between two alternatives, approximate earthwork quantities may be developed from parallax measurements on stereoscopic pairs of the photographs.
      3. To assign relative values to alternatives, it may be necessary, especially in urban areas, to secure medium scale photography, compile medium scale topographic maps, and make a tentative design in critical areas.
   4. Preliminary Survey of the selected route corridor is the stage in which information essential for preparing the construction plans, specifications, and cost estimate is obtained. The source of much of this data is aerial surveys, which may usually proceed in the following sequence. Only where tall and dense vegetation obscures the ground and precludes accurate photogrammetric measurements is it necessary to secure by ground surveys enough data to establish limits for the clearing to be done before large scale photography can be taken to complete the survey by photogrammetric methods.
      1. The basic control survey determines accurately the horizontal and vertical position of monumented points along the corridor. This survey should be of first or second order accuracy and should start and end on high order geodetic points. The horizontal position of these points should be de scribed by coordinates in the State Plane Coordinate System, datum adjusted on an area or project basis.
      2. Photographic targets are placed on all basic control points within the area to be photographed. When supplemental control is by ground survey methods, targets may also be placed on supplemental control points.
         
         Since cadastral surveying and surveying of auxiliary points to be used during construction may be included in supplemental control surveys by photogrammetric methods, photographic targets may also be placed on property corners and on auxiliary points.
      3. Large scale photography of the corridor is taken using a cartographic camera.
      4. The supplemental control survey determines the position, to the needed accuracy, of enough additional points between the basic control points to fully control the photogrammetric mapping. Supplemental control may be by ground survey, by analog photogrammetric bridging, or by aerial analytical triangulation. A cadastral survey of the corridor may be included in a supplemental control survey by photogrammetric methods. The horizontal position of auxiliary points along the route, to be used during the construction survey, may also be included in a supplemental control survey by photogrammetric methods. The cadastral survey and the survey of auxiliary points benefit from the high accuracy of horizontal position inherent in analog photogrammetric bridging and aerial analytical triangulation.
      5. Detailed cadastral and topographic maps of the corridor are prepared, using the large scale photography in photogrammetric instruments of sufficient precision to com pile maps adequate for design, acquisition of right-of-way, and preparation of construction plans. Culture and cover may be shown graphically, or orthophotographs may be used as the base for the maps.
      6. The size of drainage structures needed depends on the cover, area, topography, and length of waterways of the drainage basin. All of this information may be secured easily and quickly from the photography; many small drainage areas show on the large scale photography taken for this stage, and most of the others show on the small scale photography secured for reconnaissance.
      7. The horizontal alignment and the vertical alignment of the facility are designed to fit the environment. The designer uses quantitative information derived from the topographic and cadastral maps and qualitative information derived from stereoscopic examination of the photography and interpretation of imagery to select the final alignments. When the horizontal alignment and the vertical alignment are satisfactory, a mathematical description of the horizontal alignment and its position on the map is computed. All instrument points on the centerline are defined by plane coordinates which can be used to draw the center line on the map manuscript and to stake the centerline on the ground.
      8. To aid in designing the alignment, in preparing the engineer's estimate of cost, and for payment during construction, cross sections are measured photogrammetrically to form a digital map for computing earthwork quantities.
   5. The Location Survey delineates on the ground for construction of the complete facility as de signed, showing line, earthwork limits, rights-of-way and structure layouts. Unless preliminary clearing was necessary, this is the first staking of the facility on the ground. The location survey starts and ends on basic control survey points, and is staked using survey ties computed from the plane coordinates of the centerline instrument points and the basic control points.
   6. During the Construction of the facility, aerial surveys continue to be useful.
      1. The earthwork quantities determined photogrammetrically during the preliminary survey stage may be used for payment wherever the facility was constructed as designed. Where the vertical alignment or cross section was changed during construction, it is necessary to compute new quantities based on the previous photogrammetric measurements. Where horizontal alignment was changed, it is necessary to make photogrammetric measurements on the original photography along the new alignment. Where overbreak or underbreak occurred, or for borrow pits, photogrammetric measurements on new photography may be combined with measurements on the original photography to compute earthwork quantities.
      2. Sections of centerline needed to control construction operations may be reestablished accurately and quickly by utilizing survey ties between the centerline points and basic control points or the auxiliary points which were surveyed along with the supplemental control survey by photogrammetric methods.
      3. Aerial surveys may often be used to help solve special problems which arise during construction. Photography on hand from previous stages or new photography taken for the specific problem may be used to illustrate the problem when it is presented to others and to secure needed quantitative or qualitative information.
   7. Maintenance can utilize aerial surveys throughout the life of the facility.
      1. Vertical and oblique photography of the completed facility can supplement "as-built" plans.
      2. Periodic photographic coverage of the entire right-of-way is useful for locating and assessing erosion damage, surface condition, authorized and unauthorized en trances, and encroachments.
      3. Aerial surveys are helpful when special problems arise, just as they were during construction.