238.1 Aerial Mapping and LiDAR Surveys
| Key Points |
Photogrammetric surveys achieve the same results as conventional surveys by methods, which require a minimum of fieldwork. Photogrammetric surveys are performed by making measurements from aerial photographs properly oriented to a few field measurements to form, in proper equipment, a small scale three-dimensional model of a part of the earth's surface. Through the use of stereoplotters the location and elevation of man-made features are obtained from the model. Conventional surveys require numerous field measurements to obtain the same data and information.
238.1.1 Surveys Adaptable to Photogrammetry
Projects with certain physical characteristics are adaptable to economical photogrammetric surveys. The districts must make the judgment, based on their knowledge of the requirements of each project, whether a conventional or photogrammetric survey best satisfies their need. Factors that influence this decision include:
- Scope of the project: The use of photogrammetric methods, in almost all cases, will result in the most economical survey for projects encompassing large areas. Conversely, smaller projects are best suited for field surveys. It is difficult to provide a quantitative guideline to determine which projects fall into which category. Generally, projects less than 1300 ft. (400 m) long are best when field surveyed. Photogrammetric projects under 8200 ft. (2500 m) long can have straight line targets and any project longer than 8200 ft. (2500 m) will use the triangulation method of targeting.
- Type of project: Sometimes photogrammetry can be economically applied to resurfacing and widening projects in highly developed areas to obtain planimetry (2D only). These surveys do not require vertical control. Projects that required precise elevations are to be field surveyed.
- Terrain: Projects with terrain that is difficult or impossible to field survey, will be surveyed by photogrammetric methods. These include projects in highly developed areas, extremely rough terrain, heavily timbered areas as well as bridge surveys for large streams.
- Time consideration: Time sensitive projects will be field surveyed, because of the lead-time necessary for photogrammetric surveys. Table 238.1.1 shows guidelines to determine what type of survey best suites a project.
Table 238.1.1
| Photogrammetric Survey Versus Conventional Survey | |
|---|---|
| Photogrammetric | Conventional |
| Wide Corridor Projects | Resurfacing Projects |
| Relocation Shoulder | Widening Projects |
| Large Area Planimetric (2D) Surveys | Small Area Planimetric (2D) Surveys |
| Large Bridge Replacements | Small Bridge Replacements |
| > 1300 ft. (400 m) | < 1300 ft. (400 m) |
| Interchanges | - |
| Rough Terrain | - |
In addition, district personnel that request mapping, will discuss project specifics with the photogrammetry personnel when determining which method of survey best suits the project.
238.1.2 Stages of Photogrammetric Services
Photography and compilation for photogrammetric surveys are centralized in Design since these require the use of expensive equipment that is only economical if fully utilized. All other work is performed in the district. The following sequential stages for a photogrammetric survey must be coordinated between the district and photogrammetric personnel:
1. Recommendations for flying program
2. Mapping and Photography limit plan
3. Flight planning
4. Target planning
5. Targeting
6. Photography
7. Establishing control
8. Compiling photogrammetric survey data
9. Furnishing electronic data to the district.
The district performs Stages 1 and 2 on all projects, and in some cases may perform Stages 5 and 7.
238.1.3 Recommendations for Flying Program
The district recommends projects for the flying program each year at the request of Design by the last day of September. Projects recommended for mapping are those that a location study, if necessary, has been approved, or projects that this will be completed in time to obtain the photography during the flying season (approximately December 15 to April 15). Normally, projects are in the design year of the approved STIP are considered, but other projects may be considered if conditions warrant. Other projects, flown only for aerial photo coverage in preparation for a location study, will also be considered at this time.
238.1.4 Flight Planning
Design performs flight planning, but information from the district is necessary to efficiently plan the photographic mission. The district furnishes information including the location of the proposed improvement indicated in a MicroStation Design file, and the desired mapping scale. Mapping and Photography limits are to be submitted electronically using MicroStation and ProjectWise.
It is desirable to limit the photogrammetric corridor to only that area which is necessary for the design of the project. Separate corridors, planimetric (2D) and terrain (3D) can be described for photogrammetric surveys. The district's recommendation regarding the type and extent of photogrammetric survey data coverage will consider the following:
- For planimetric (2D) coverage, corridors will include all features that may affect design considerations and right of way takings. Planimetric corridors do not have to be connected but must be within the area in which horizontal controls have been established.
- For terrain (3D) coverage, corridors will be limited to the area necessary for earthwork computations. Generally, this area is within the limits of proposed right of way. Terrain corridors do not have to be connected but must be within the area in which horizontal and vertical control have been established.
- Generally, corridor requests do not include areas for drainage computations. Keep in mind that photogrammetric data should be supplemented with conventional survey data, and shall be verified by the district survey party.
238.1.5 Theory of Flight Planning
There is a direct relationship between the flight height, photo-coverage, focal length of the camera, and the size of the negative, which is taken into consideration when planning a photographic mission. As the flight height increases, the photo-coverage increases and the scale of the negative becomes smaller. Where the photogrammetric mapping is to be performed to a predetermined scale, the flight height, and consequently the photo-coverage on any one negative is predetermined by the camera and stereoplotter being used. On low-level photographic missions or where mapping of relatively large areas is desired, it is sometimes necessary to plan extra flights to obtain coverage of entire areas. Photographic missions cannot be properly planned without knowledge of the intended use of the photography or the scale desired for the mapping. The focal length of the department's camera is 150 mm (5.9 in.) and the photo is 230 mm (9 in.) square. Whereas the focal length of the digital mapping camera is 120 mm (4.7 in.) and the photo coverage is 92 mm x 166 mm (3.6 in. x 6.5 in.).
A summary of common flight heights is shown on Tables 238.1.5.1 and 238.1.5.2. The accuracy of the mapping compiled by photogrammetry personnel is in direct relationship to the flight height. Photography flown at an altitude of 1500 ft. (460 m) above mean sea level has a mapping accuracy so that 90% of the elevations will be within 6 in. (150 mm). Photography flown at an altitude of 3000 ft. (920 m) above mean sea level has a mapping accuracy so that 90% of the elevations will be within 12 in. (300 mm).
Table 238.1.5.1
| Photogrammetric Relationships (Frame Camera) | ||||
|---|---|---|---|---|
| Flight Height AMG (ft.) | Photo Cover (ft.) | Photo Scale (ft./in.) | Min. Contour Interval (ft.) | Accuracy (ft.) |
| 1500 | 2264 x 2264 | 250 | 1 | 0.5 |
| 3000 | 4528 x 4528 | 500 | 2 | 1 |
| 6000 | 9055 x 9055 | 1000 | 5 | 2.5 |
| 12000 | 18110 x 18110 | 2000 | - | - |
Table 238.1.5.2
| Photogrammetric Relationships (Digital Mapping Camera) | ||||
|---|---|---|---|---|
| Flight Height AMG (ft.) | Photo Cover (ft.) | Photo Scale (ft./in.) | Min. Contour Interval (ft.) | Accuracy (ft.) |
| 1181 | 907 x 1633 | 250 | 1 | 0.5 |
| 2363 | 1841 x 3266 | 500 | 2 | 1 |
| 4725 | 3628 x 6532 | 1000 | 5 | 2.5 |
| 9450 | 7256 x 13064 | 2000 | - | - |
238.1.6 Authority to Target
Design consults the targeting for the flight program, however the district may perform their own targeting. If the district decides to perform the targeting, the target locations will be provided by Design in a MicroStation design file and coordinate file. Before targeting the location of the improvement, the district must check with Design to ensure the plane will be available for photography as scheduled.
238.1.7 Targeting and Photogrammetry
If the the targeting is performed by the district, the district will advise Design when targeting is complete targeting is complete so the location can be photographed as soon as possible. The district is furnished one set of contact prints of the aerial photographs. The district will use the contact prints to indicate the target locations and additional vertical control if needed.
238.1.8 Vertical Control
As soon as practical after completing the photography, the district will be furnished with contact prints to plan for obtaining vertical control, if needed. The district then proceeds with the survey to obtain vertical control. When the vertical control survey is completed, the district returns the vertical control photographs along with the vertical control field notebook to the Design Division.
238.1.9 Vertical Control Elevations
The district is furnished with photographs showing the required vertical control points. The location of these points is marked with a red circle and a 900 series number on the front of the photographs. The Design Division places a description of the points on the back of the photographs. The district is required to provide the described point elevation with the book and page number of the respective survey field book notes. Examples of these points are targets, centerline of the road at entrances, fence corners, sidewalk intersections, top of manholes and "ground 10 ft. east of lone tree". The person picking the point on the photograph uses a pocket stereoscope since it is important to select the exact location. A number on the front of the photograph identifies these points. Level lines for establishing the elevations are turned through the control point. A complete set of level notes is kept in a separate notebook. The point identification number and description for each vertical control point elevation is identified in the notebook. The elevations of the points, along with the notebook page and number used for recording the elevation are recorded by the field personnel on the back of the photograph in the space provided. The district adds the names of important streets and roads appearing on the photographs.
238.1.10 Datum
All elevations and vertical control are based on USGS or NGS datum.
238.1.11 Distribution of Aerial Photographs to the District
Digital images will be furnished to the district for retention in addition to the contact prints provided for vertical control and corridor delineation. An OrthoMosaic will be furnished to the district also.
238.1.12 Identification of Aerial Photographs
A series of numbers appear along the leading edge of all aerial photographs for identification. A photograph labeled as "96 67 1:6000 FEB 2, 1986 265 1 109" is a photograph in St. Louis County (96), on Route 67, flown at a photo scale of 1:6000, on the specified date, located on roll 265, flight 1, exposure 109 (prior to 1987, the label format was county, route, roll, flight, exposure, scale and date). The flight number and exposure number is the portion of the photograph identification number used by survey parties as a reference in their field books. A duplicate print of any aerial photo may be ordered by specifying the roll number and the exposure number. In addition to these numbers, each photograph contains other marginal information such as number of camera exposures, time of exposure, and altimeter reading, that are for the use and reference of the photogrammetry section.
238.1.13 Horizontal Control
Horizontal control for photogrammetric surveys is defined as the field control required to orient the photographs to a datum line that may be either the survey centerline or a random traverse line. The preferred method is to establish a random base or traverse line that is tied to photo-identifiable objects or random targets and from which the final survey centerline is computed. Another method is the establishment of the survey centerline in the field with targets placed on the centerline prior to photography so the targets are visible and identifiable on the photographs, thereby delineating the location on the photographs. Another method is the establishment of the survey centerline after the photography and tying the centerline to photo-identifiable objects or random targets. The preferred method is used where it is impractical to survey the centerline because of obstructions such as extensively developed areas or where the best location is not apparent until after the mapping is complete. The second method is used when the location is definite and the project completion schedules will allow the centerline to be established prior to the photography. The third method is used on short projects when there is not sufficient time in the project completion schedule to establish targets prior to photography.
238.1.14 Post-Flight Horizontal Control
Targets are planned to minimize Post-Flight Control, but in some cases where a target may have been destroyed or disturbed, Design will request Post-Flight Control. This requires photo-identifiable objects be tied to a traverse line. Horizontal control points are indicated by a red triangle with an 800 series number on the front of photographs furnished to the district for horizontal control. A description of the points is included on the back of the photograph. Both horizontal and vertical control points may be indicated on the same photograph. Examples of these points are the center of manholes, sidewalk intersections, pavement joint or intersections, power poles or other photo-identifiable objects. The survey party establishes the position of these points in the field, references them to the centerline or to the base traverse. The position includes an accurate angle and horizontal distance to the base traverse or the state plane coordinates of the point. The horizontal control is planned before the centerline or traverse line is established in the field and the survey party leaves recoverable horizontal control points near the probable centerline of the roadway.
238.1.15 Accuracy
At least second order survey accuracy is necessary. The acceptable accuracy tolerances for second order surveys are shown in Table 238.1.15. The accuracy of any traverse is not known until it is closed. The traverse may be closed on itself by looping or by tying the ends of the traverse to points on another traverse or triangulation network of as high or higher order. A second order traverse may be tied to a first or second order network, but not to a third order network. Most high order monuments were established by the NGS or the USGS. However, some cities have monuments established by first or second order surveys. Descriptions, state grid coordinates, geodetic locations and grid and geodetic azimuth are obtained for these monuments by contacting the proper governmental agency.
Table 238.1.15, Tolerances
| Second-Order Class II Survey Accuracy | |
|---|---|
| Measurement | Tolerance |
| Distance | 1:20,000 |
| Angular | |
238.1.16 Targeting
Targets may be placed any time after the location of the proposed improvement is established. When targets are placed before the centerline is staked, the targeted points are established and referenced so the target locations can be accurately re-established and their station location determined when the centerline is staked. Targets are the preferred method of making photo identifiable points to which horizontal and vertical datum can be referenced to control the mapping project. The targeted points are to be established with care to ensure they are in useful locations with clear fields of view on the ground to aid future reference to centerline survey control. Targets are located on level ground with the sky unobstructed 10 degrees above the horizon so satellites are visible to the global positioning system (GPS) receivers. It is preferred to locate targets out of traffic as the GPS receivers may have to occupy the target for 90 minutes at a time. The surveyor may begin collection of horizontal and vertical control data before the area is photographed. Aerotriangulation is a process to supplement the horizontal and vertical control normally required to control aerial photographs. Measurements of angles and distances on overlapping photographs are related into a spatial solution using the perspective principal of the photographs. Methods of aerotriangulation allow the photogrammetrist to supplement the field control data by an interpretative process to reduce the amount of field control required to properly orient the photo models. Those mapping projects that are of greater length than four photographs can have the field control requirements reduced significantly by the use of aerotriangulation. Projects which are several miles (kilometers) in length can have the field control reduced to one third of that required by a fully controlled job with the use of aerotriangulation. Therefore, with limited field control it becomes even more critical to accurately locate and identify the targeted control points.
238.1.16.1 Material for Targets
White paint or reflective white marking tape is used for targets on paved surfaces. Unbleached muslin is used for targets on grass, dirt or aggregate surfaces. Paint is available in the district while marking tape and muslin is stocked in the central warehouse for requisition by the district.
238.1.16.2 Spacing and Placement of Targets
Rough target locations will be provided by Design in both MicroStation design and xy coordinate fiels. Targets are to be spaced in accordance with specific guidelines. The distances are a function of the flight height and the desired contour interval. All of the distances for target spacing may be adjusted by ten percent to allow proper placement in the field. The mapping project must begin and end with three horizontal and vertical control targets that are roughly placed in a triangular pattern. The two lateral targets are spaced at the offset distance with the third target placed near the mapping corridor. No mapping will be done beyond the last target; therefore, enough targets must be placed to insure adequate coverage. Position targets in locations with a good field of view to minimize the cutting of vegetation and reduce the number of required ground setups. Targets are located as required for visibility from the air in areas free of shadows. When targets are placed on paved shoulders of the roadway, it is suggested the northern shoulder be used to avoid obscuring the target with shadows from objects on the southern side of the road. Painted targets on pavement, located within public right of way, are preferred to cloth targets located on private property. When cloth targets are placed, they must be located on level areas with all underbrush and weeds removed adjacent to the targets. Targets are located where they are least likely to be disturbed. Targets are placed so the time lapse between placing the targets and the photography is held to a minimum. If the time lapse is of such duration that it may cause doubt as to the target condition, the targets are checked immediately prior to photography.
238.1.16.3 Field Notebooks (Targets)
A separate notebook is used to record the target locations and descriptions. The notes include, for each target, the target numbers, shape of the target, and the target ties. A sketch should be provided in the field notes so someone who is not familiar with the location can identify each target on the photographs and easily relocate the targets in the field. Offset distances are recorded for targets that are offset. The targets are listed in numerical sequence in the field book. Acceptable examples for recording target notes are available.
238.1.17 Base Traverse Surveys
A high order accuracy base traverse is established after photography for use as horizontal control and for use in computing the centerline location. This traverse is established with electronic distance measuring equipment and a theodolite, a total station or a GPS unit.
238.1.18 Traverse Adjustment
Actual trial field surveys have been conducted to determine the most efficient methods for second order accuracy surveys. A computer program has been written based on these methods for the adjustment of traverses that practically eliminates the need for manual calculations. Use of this computer program requires the following methods and procedures be used:
- Establishing traverse stations. After the general location of the base traverse is established, the traverse stations are selected. Traffic, parked cars, and the location of objects, which might cause refraction, are considered. Once the location of the stations is determined, monuments are erected and referenced. The monument is of a material not subject to easy deterioration or movement, like a cross in concrete or 100d spike driven below the ground line. The traverse stations are located in areas with easy access having good fields of view that will not be disturbed by normal activities until the centerline can be staked.
- Turning angles. If practical, angles are turned at the time of day best suited for this type of work. Early morning offers the advantage of lack of heat waves and little refraction. When angles are being turned under a hot sun the instrument must be shaded. In urban areas traffic volume may dictate the best time of day for turning angles. Range poles are used on long sights while plumb bobs are used on short sights. A tripod is used to hold them over the station. The range pole is plumbed using a tribach with optical plummet. The instrument is carefully leveled and centered over the station. Traverse angles are turned eight times, four direct and four reversed, using the mean angle. Four angles are turned to sub-points, two direct and two reversed, with the mean value used. Angles to mapping targets are turned twice, once direct and once reversed, again with the mean angle used. When a theodolite is used, vertical angles are also measured to convert the slope distance measured by the electronic distance measuring equipment to horizontal distance.
- Measuring distance. Distances are measured with an electronic distance measuring instrument (EDM) or a total station. Distances are measured to reflectors that are placed on a tripod and centered over the survey point. Each distance to a traverse station or sub-point is measured four times. Distances to a mapping target are measured twice. In each case the mean of the measured distance is used. In the case of an EDM, which is a slope distance, the measurement must be converted to a horizontal distance.
- Field notebooks (traverse). Acceptable examples for traverse notes are available.
- Polaris and Sun observations. To properly close some traverses it is necessary to obtain the correct bearing or azimuth of some of the lines involved by Polaris observation. Computations for second order observations of Polaris at any hour are found in any ephemeris. There are two important considerations in the field procedure of a Polaris observation:
- The theodolite must be level
- The time must be correct
The theodolite must be in adjustment with good workmanship performed by the instrument operator. The correct time is obtained by radio from the Bureau of Standards, 2.5, 5.0, 10.0, 15.0 and 20.0 megacycles, the Naval Observatory, 113 kilocycles (five minute period every hour), or the Canadian Observatory, 7.5 megacycles. If a portable short-wave radio is available it is taken to the field. If not, a watch used is checked before and after the observation. Field procedures used to give a second order Polaris observation are as follows:
- The theodolite is set over the traverse station and leveled carefully. With the telescope, direct a back sight on the traverse station at the other end of the line being checked, with the circle reading read and recorded.
- The telescope is turned to the star and the instant that the star passes behind the cross hair is noted. The time is recorded to the nearest second, along with the circle reading.
- This procedure is repeated four times with the telescope direct, and four times with the telescope reversed.
- Each angle and time is used to compute a bearing of the survey line. The mean of these eight computed bearings is used.
An acceptable example for recording a second order Polaris observation is available. Sun observations are an additional way to establish bearings. Using an ephemeris, the correct time, and following procedures for correctly sighting the sun, bearings can be established. This procedure may not be as accurate as GPS or a Polaris observation, but can be used as another tool by the surveyor.
- Vertical control. The elevation of three or more vertical control points is necessary on each photograph to orient the photograph vertically in the stereoplotter. Aerotriangulation permits the photogrammetrist to reduce the amount of required ground control elevations for an entire project but proper ground elevations are still required to obtain reliable surface elevations.
- Benchmarks and centerline profile. Benchmarks are established prior to obtaining the vertical control. An elevation is obtained at each target. The centerline profile is taken from the aerial contours. After the designer establishes the approximate location of culverts using the aerial contours, culvert sections are taken in the field.
- Extra elevations. In running levels from the established benchmarks to the vertical control points, the survey party is encouraged to obtain elevations for additional photo-identifiable points that may be useful. If a point selected by the photogrammetry section cannot be identified in the field, the elevation of one additional point is obtained on each side of the unidentifiable point in the direction of the line of flight. The location of the additional points is marked with a cross on the photograph, pin pricked with the elevation and description recorded in the field books and on the back of the photographs.
- Field notebooks (vertical). Vertical control field notes are recorded. Field books used in obtaining the vertical control are forwarded to Design with the vertical control photographs and the target elevations as soon as the fieldwork is completed. All notebooks are returned to the district when the plotting is completed or at any time on request.
238.1.19 Aerial Photogrammetry
The photogrammetry section is available to perform various photographic assignments upon request. In order to properly plan the photographic mission, the Design Division is furnished with basic information regarding the desired photography together with the request for the photography. Aerial photographs are also available for distribution within MoDOT. Generally, photographs are not furnished to others nor is photography work performed for others. However, exceptions may be made for special circumstances.
238.1.20 OrthoMosaics
The photogrammetry section will provide the district with OrthoMosaics on every mapping project. An OrthoMosaic is spacially correct mosaic, which includes the photos for the entire project. The mosaics are generated at 0.5 ft. per pixel. The district design personel can use the mosaics as raster images behind the project corridor line work prepared using CADD tools for the purpose of public displays.
238.1.21 Requisitioning Photography and Photographs
A request for contract prints and enlargements is made using Form D-102.