238.1 Aerial Mapping and LiDAR Surveys
From Engineering Policy Guide
Aerial Mapping and LiDAR surveys can achieve the same results as conventional topographical surveys by methods which require a minimum of fieldwork. Airborne LiDAR sensors are used by companies in the remote sensing field. It can be used to create DTM (Digital Terrain Models) and DEM (Digital Elevation Models). This is a common practice for larger areas since a plane can take in a swath 1 km wide in one flyover. Greater vertical accuracy can be achieved with a lower flyover and a narrower swath, even over a forest, where the height of the canopy as well as the ground elevation can be determined. Conventional surveys require numerous field measurements to obtain the same data and information.
238.1.1 Surveys Adaptable to Aerial Mapping and LiDAR
Projects with certain physical characteristics are adaptable to aerial mapping and LiDAR surveys. The districts must make the judgment, based on their knowledge of the requirements of each project, whether a conventional or aerial mapping and LiDAR survey best satisfies the need. Factors that influence this decision include:
- Scope of the project: The use of aerial mapping and LiDAR methods, in almost all cases, will result in the most economical survey for projects encompassing larger 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, it is better to field survey projects that are shorter than 1300 feet. Aerial mapping and LiDAR projects should be reviewed to determine the best application of LiDAR that will meet the project needs and budget.
- Type of project: The project's scope will usually be the best guide for the type mapping needed. Urban enhancements of existing structures may lend themselves to a terrestrial or mobile type of collection for the best results. Where a cross country new alignment would be much better suited for aerial platform like a fixed wing aircraft or helicopter.
- Terrain: Projects with terrain that is difficult or impossible to field survey will be surveyed by aerial mapping and LiDAR 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 aerial mapping and LiDAR surveys. Table 238.1.1 shows guidelines to determine what type of survey best suites a project.
|Aerial Mapping and LiDAR Survey Versus Conventional Survey|
|Aerial Mapping and LiDAR||Conventional|
|Wide Corridor Projects||Resurfacing Projects|
|Relocation Shoulder||Widening Projects|
|Large Area Planimetric Surveys||Small Area Planimetric Surveys|
|Large Bridge Replacements||Small Bridge Replacements|
|> 1300 ft.||< 1300 ft.|
In addition, district personnel requesting mapping should discuss project specifics with the aerial mapping and LiDAR personnel when determining which method of survey better suits the project.
238.1.2 Stages of Aerial Mapping and LiDAR Services
Data acquisition for aerial mapping and LiDAR surveys are provided by professional consulting services. These professional consultant services are managed by Design Division. These services are provided via the “Annual Flight Program Contract” and the following sequential stages must be coordinated between the district and Design Division:
- 1. Recommendations for the flight program
- 2. Mapping limit plan
- 3. Accuracy planning
- 4. Consultant selection
- 5. Quality control on projects
- 6. Review and quality checks on aerial mapping and LiDAR survey data
- 7. Furnishing electronic data to the district.
188.8.131.52 Recommendations for Flight Program
The district recommends projects for the flight 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 for which this will be completed in time to obtain the data during the flying season (approximately December 15 to April 15). Normally, projects that are in the design year of the approved STIP are considered, but other projects may be considered if conditions warrant.
184.108.40.206 Flight Planning
Design performs flight planning, but information from the district is necessary to efficiently plan the the aerial mapping and LiDAR mission. The district furnishes information including the location of the proposed improvement indicated in a CADD drawing file, and the desired accuracy. Mapping and photography limits are to be submitted electronically using CADD software and ProjectWise.
It is desirable to limit the aerial mapping and LiDAR corridor to only that area that is necessary for the design of the project. The district's recommendation regarding the type and extent of aerial mapping and LiDAR survey data coverage will consider the following:
- For planimetric 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 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 aerial mapping and LiDAR data should be supplemented with conventional survey data and shall be verified by the district survey party .
220.127.116.11 Accuracy Planning
To best extract topographic and manmade features from the LiDAR data, three density and accuracy levels are used, as describe below:
- Type A, Roadway and Pavement Scans (Mobile, Helicopter Based or Terrestrial LiDAR)
- 1) Internal Horizontal / Vertical Accuracy of 0.3 ft. at 95% confidence.
- 2) Maximum point spacing of 0.3 ft. on the full classified LAS file.
- Type B, Corridor and earthwork Scans Urban (fixed wing or Helicopter Based Aerial LiDAR)
- 1) Internal Horizontal / Vertical Accuracy of 0.5 ft. at 95% confidence.
- 2) Maximum point spacing of 1 ft. on the full classified LAS file.
- Type C, Corridor and earthwork Scans Rural (fixed wing)
- 1) Internal Horizontal / Vertical Accuracy of 0.5 ft. at 95% confidence.
- 2) Maximum point spacing of 2 ft. on the full classified LAS file.
18.104.22.168 Consultant Selection
22.214.171.124 Quality Control on Projects
Quality control all projects will be done by the Central Office Design survey staff to insure the data provided meets MoDOT’s needs for engineering mapping and design.
To verify the accuracy of the surface of the delivered aerial mapping and LiDAR, the Central Office survey staff will take check shots along the main alignment of the project at least every 200 ft. and alternate shoot every 400 ft. each side of the main alignment.
To verify the accuracy of extracted features of the delivered aerial mapping and LiDAR, the Central Office survey staff shall take check shots along curb lines, bridge rails, retaining walls and other features with elevation differences that can easily be identified.
126.96.36.199 Review and Quality Checks on Aerial Mapping and LiDAR Survey Data
All quality control will be done using MoDOT's CADD software. Project control shots will be compared to the surface model and topographic and manmade features provided by the aerial mapping and LiDAR consultant and must meet the internal horizontal/vertical accuracy defined by the scope of services at 95% confidence.
A report will be run and kept with the files for the project certifying these accuracies.
Any acceptance of data below these standards will have written documentation explaining the prescence of this data and what resulting developments may be encountered during the design process.
188.8.131.52 Furnishing Electronic Data to the District
All data will be delivered to the districts via ProjectWise. Once all the data has been delivered and has met the quality standards, Central Office will place the file into the districts' ProjectWise folders for access.
238.1.3 Standard Deliverables for LiDAR Data
The consultant shall provide to the Commission the following items:
- 1) Three ASCII coordinate files all containing the primary control, photo control and check points for the project survey. These files are:
- Primary Control File. A file listing control positions by point number, X, Y, and Z values in project units referenced to the Missouri Coordinate System of 1983, the correct zone name (ex. East Zone), with X and Y values modified by the projection factor. This ASCII formatted file will be in the form of: J######.rec.
- The Geodetic Control File. A file containing latitude and longitude information for all control points named J######.txt with file format listed on page 3 of Fig. 238.1.3, Aerial Photography and Control Survey. All OPUS solution sheets and/or data sheets from post processed static GPS sessions, calculations for grid and projection factor including the centroid point, mean elevation and the final grid and projection factor will also be listed in this file.
- Check Shots File. A file listing quality control positions by point number, X, Y, and Z values in project units referenced to the Missouri Coordinate System of 1983, the correct zone name (ex. East Zone), with X and Y values modified by the projection factor. This ASCII formatted file will be in the form of: J######.txt.
- 2) MoDOT Survey Report. A MoDOT survey project report for each project. It shall include copies of all inter-visible control survey pair station descriptions along with all benchmark descriptions and field ties. A sketch of each point shall be provided showing the relative location of field ties to the point being referenced. The consultant shall provide a letter certifying that the below mentioned surveying specifications have been achieved for this project. The letter shall document the relative positional accuracies in parts per million, the confidence level in percent and the post adjustment residual values in centimeters that were achieved on this project. If any portion of the survey does not comply with these specifications, a written report substantiating the material variances from the specifications with the responsible surveyor’s signature is required. The Commission reserves the right to disallow variations.
- The survey report documents proof of these specifications:
- a. Fixed preprocess baseline solutions.
- b. Control station relative positional accuracies of 10 ppm in relation to adjacent stations at the 95% confidence level.
- c. Post adjustment residual values <3 cm in any dimension for control stations.
- d. A dgn file with all survey control points plotted and labeled.
- 3) An Orthomosaic captured simultaneously with LiDAR or separate aerial sensor, meeting the following requirements:
- a. Shall have a resolution of 0.5 ft. per pixel.
- b. Shall be tiled, with tiles no larger than 3250 x 3250 pixels.
- c. Shall encompass the area requested for mapping.
- d. Shall be a geotiff and accompanied by a projection file (.prj).
- e. Shall include a shape file indicating the locations of the orthomosaic tiles.
- 4) LiDAR projects, the following shall be delivered:
- a. Data will be delivered in LAS version 1.2 format or newer with the following information.
- i. Record return
- ii. Intensity
- iii. GPS time
- iv. Swath line number designation
- v. Classification values after trimming (without data voids between swath lines)
- 0 = raw, never classified
- 1 = unclassified
- 2 = ground (i.e. bare earth)
- 3 = low vegetation
- 4 = medium vegetation
- 5 = high vegetation
- 6 = building
- 7 = low point
- 9 = water
- 10 = bridge
- 12 = overlap
- b. LiDAR Processing Report.
- c. Vertical Accuracy Report.
- d. A shape file containing numbered LAS tiles.
- 5) ASCII coordinate file for each project, containing the following items for each point:
- a. X, Y, and Z coordinates using the Missouri Coordinate System of 1983, the correct zone name (ex. East Zone), modified by a factor developed by the consultant.
- b. Feature code using MoDOT Standard Surveying Feature Codes.
- 6) Microstation and Geopak files to be provided:
- a. Provide a Topo_ConsultantName_JOB#.dgn (3D MicroStation file) of all the topographic and manmade survey data collected.
- i. All dgn files will be based on modified state plane coordinates, using the projection factor for the project as described in EPG 238.1.4 Datum and Horizontal Control.
- ii. Working units: U.S. Survey Foot
- iii. Features shall be plotted according to MoDOT CADD Standards. Features to be plotted at 1” = 100’ scale. Standards are available in the GEOPAK Survey Manager Database (.smd).
- Topography Features. The mapping data shall include natural positions on the earth’s surface within the project limits that determine the configuration of the terrain. The positions will be in the form of points and strings that locate vertical and horizontal transitions.
- Planimetry Features. The mapping data shall include the positions of all natural and all man-made features within the project limits. The positions will be in the form of points and strings that define the shape, size and position of the features.
- b. Tin and GPK files will be based on modified state plane coordinates, using the projection factor for the project as described in EPG 238.1.4 Datum and Horizontal Control.
- c. Geopak Digital Terrain Models (.tin) for the entire project. Tin models should not exceed 200 megabytes.
- d. Geopak Coordinate Geometry Database (.gpk) containing the data imported for the project. gpks should not exceed 30 megabytes.
238.1.4 Datum and Horizontal Control
184.108.40.206 Linear measures
Linear measures will be made in the English System. The base unit will be the United States Survey Foot (and decimal parts thereof).
220.127.116.11 Coordinate System
All coordinates shall be based on the State Plane Coordinate System, North American Datum (NAD) of 1983 (1997) in the appropriate zone for the project.
18.104.22.168 Vertical Datum
The elevations shall be based on the North American Vertical Datum (NAVD) of 1988. The elevations shall be based upon ellipsoidal heights that have been modified by the most current NGS Geoid model.
22.214.171.124 Projection Factor
The consultant is responsible for developing a project projection factor based on the Missouri Coordinate System of 1983 Manual for Land Surveyors.
Scale Factor. Using the most easterly and westerly control points within the project to develop a centroid point for a project. Use the converted English easting of the centroid point in the correct zone formula, below.
Elevation Factor is determined by dividing the ellipsoid radius by the ellipsoid radius plus the mean elevation for the project.
Grid Factor is the result of multiplying the Elevation Factor by the Scale Factor of the centroid point of the project.
- Grid Factor = Elevation factor * Scale factor
Projection Factor is the reciprocal of the grid factor.
- Projection Factor = 1 / Grid factor
238.1.5 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.
238.1.6 Aerial Mapping and LiDAR MOU
When mapping becomes necessary beyond the time frame of the annual flight program, district may work with the Central Office Design survey staff to hire an on-call consultant to perform these services. Please refer to EPG 134.2.4 Consultant Solicitation and Selection Process – Standard Solicitation Method for On-Call Cost Plus Fixed Fee Contracts.
Orthomosaics are a standard delivery with aerial photography mapping. An Orthomosaic is a spacially correct mosaic, which includes the photos for the entire project. The mosaics are generated at 0.5 ft. per pixel. The district Design personnel can use the mosaics as raster images behind the project corridor line work prepared using CADD tools for the purpose of public displays.
238.1.8 Requisitioning Historic Photographs
A request for contract prints and enlargements is made using Form D-102.