Chapter 9 - Remote Sensing

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9.1 Static Laser Scanning Standards

As laser scanning evolves over time, these standards will be modified to reflect the adoption of advancements and more efficient procedures. The following was developed with no brand of scanner in mind. Until the industry settles on standard format, MDOT has a generic formatted deliverable. The following guidelines should be followed in order to obtain satisfactory results for MDOT standards. It is the Project Surveyor’s / Consultant’s responsibility to discuss targeting, merging, registration, geo-referencing, QA/QC checks, and importation into CADD/Design software of the scans with the MDOT Survey Consultant Project Manager for the project or Region Survey Manager before work begins.

  • It is important for the project surveyor to keep in mind that the ultimate goal is to use the scan data to produce an accurate survey DTM that is representative of actual field conditions. Scan data shall be exported into a format that may be used to prepare the DTM surface and Planimetric Data. Scan data and 3D models will be considered as supplements to, and NOT replacements for the required MDOT CADD/Design software deliverables.
  • All surveys for control and geo-referencing to support and establish STLS surveys by and for MDOT must meet accuracy standards and follow approved methods as described in the “Control’ section of this document. Following these standards helps ensure the product deliverables meet MDOT requirements.

It is not MDOT’s intention to limit a company’s internal practices or the innovation of a company. Input from scanning consultants may be considered for a project, at MDOT’s discretion, to deviate from these guidelines. If appropriate, and at MDOT’s discretion, changes and new methods tested on projects with a good result may be placed in the next update of the standards. All deviations from these standards are required to be explained in the scanning report at the time of submission of the scan data for review.

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9.1.1 Scanning Project Folder Requirement Information

  • When scanning is used for data collection, all scanning data must have a separate folder and not a section in the main job portfolio. See Chapter 10 - Deliverables for an example of the folder structure
  • A scanning report will be placed in the Scan folder with any notes, or deviations from the standards. The surveyor's report in the job portfolio must still state that scanning was used but does not need to be elaborate. A Scanning Report shall include at minimum:
    • The equipment Model, Type, Serial Number, Software, and Version.
    • A discussion about data tolerance issues and errors in creation of DTM, from the registration of the scans and geo-referencing from measured QA/QC checks.
    • Any information pertaining to scanning that will eliminate possible questions.
  • Provide a network diagram showing the location and label of each control target, Scanner setup, and related metadata.
  • A Scanning Control Point list will be created for Control points used for the scanning project not considered or included in Primary/Intermediate Control list. This file must be an ASCII (P, N, E, and Elev) and associated Std Deviations for each control point must be provided.
  • A copy of the registration residuals between control points of adjacent scan setups must be included in the report as well as any cloud-to-cloud registration residuals (Adjustment report from scanning software).
  • Provide an NSSDA type report showing the 95 % results comparing validation points to the final processed/registered point cloud. Validation Points will be evenly dispersed throughout the project extents on hard surfaces.
  • Provide an NSSDA type report showing the 95% results comparing validation points to the final CADD / Design software DTM surface to be provided for design.
  • Documentation of any changes from standards approved by the MDOT Survey Consultant Project Manager or other approved MDOT employee.
  • A copy of the scan data must be provided on a USB high-speed peripheral device in format of e57, or LAS/LAZ. Depending on the size and nature of the project, acceptable formats will be delivered as either a combined point cloud, individual scans or tiled data (with associated tile reference index). Delivery format should be discussed with the MDOT survey consultant project manager or MDOT region surveyor.

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9.1.2 Scanning Data Requirements

  • Individual points should have a final scan density spacing not exceed 0.1 ft. on structures and not exceed 0.3 ft. on road surfaces.
  • The scanner setups should be spaced accordingly with respect to its published capabilities for 3D accuracy and range. (Scanner dependent and provided Item 2.a is met.)
  • Scan setups must overlap by a minimum of 20% if using Cloud to target Registration only. More overlap will be required for cloud to cloud registrations.
  • Environmental conditions (rain) and material being scanned affects the scan quality. Wet pavement conditions and low reflective materials (new bituminous) must be avoided, or will require the maximum distances measured from the scanner to be shortened to meet the minimum density requirement.
  • All areas scanned must have an associated high quality digital photograph overlaid on the scans. Note: digital photographs need to be taken in some instances multiple times to eliminate excess obstructions of the area. (i.e. traffic and vehicles).
  • It is recommended that fixed height tripods with standard targets be used for scanning.

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9.1.3 Scanning Control and QA/QC

  • All scan projects must have a minimum of 4 control points.
  • All control points must be established via acceptable methods described in Chapter 4 of the Survey Standards of Practice.
  • Elevations to control points must be established in accordance with Chapter 4.1.2.5 Vertical control, and on the appropriate vertical datum.
  • Scan control must be numbered differently than Primary and Secondary control to show geo-referencing points in QA/QC.
  • Control points for geo-referencing shall be set as needed to ensure horizontal and vertical project tolerances are met.
    • All the positions of the control points list shall be provided.
    • The targets shall be located such that targets do not form a linear pattern. It is intended that the targets be properly spaced within the scan world to control the tilt and elevations of the scan and provide the necessary geometric strength.
    • Exceptions:
      • Any additional ground targets used to aid in geometry of registration of scans, must have coordinates published in the scanning control point list.
      • Magnetic targets can be placed on bridge structures to aid in registration. Coordinates for these items are optional.
      • Temporary marks (ie. paint, lumber crayons [keil], or permanent markers) of any kind are not approved in road line applications
      • Additional Tie points for registration are always recommended.
Control points used for scan targets must be monumented in the field with:
  • Using Main Project Intermediate Control (#5x 36” rebar)
  • 18” long #4 rebar rod with a plastic cap
  • Magnetic Survey Nails (Use in limited access areas)
    • Minimum length in Asphalt is 1/2”
    • Minimum length in Concrete is 1/4”
Uses of Intermediate Control in scans:
  • Intermediate Control shall be used in all terrestrial scanning applications, and each be used in a minimum of two different scans.
  • At minimum of 20 hard surface check observations need to exist. Check shots via total station or RTK GPS with a local base on adjusted MDOT control, must be provided to verify the point cloud and must be located evenly throughout the project. The observations do not need to be on a given line and can reside on a PAVED shoulder or any hard surface that exist in the DTM.
  • In linear projects greater than 0.5 miles or 2640 feet, a hard surface check shots should be spaced every 100 feet or less.
  • The check observations need to reside in a separate segments labeled in the description as “Scanning QA/QC.”
  • All check points are to be coded as “CHK” and be verified they are not part of the DTM surface. Validation elevations should be within 0.05 feet of the final adjusted point cloud.

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9.2 Mobile Terrestrial Laser Scanning Standards

The intent of this document is to provide guidelines and standards related to the use of Mobile Terrestrial LiDAR (MTL) on design engineering projects for the Michigan Department of Transportation. These standards and guidelines are also intended to provide the methodology and procedures that will help ensure products produced with MTL will meet MDOT's needs for design engineering for transportation corridors.


9.2.1 MDOT MTL Project Information

  • Mobile Terrestrial LiDAR (MTL) (mobile mapping or mobile laser scanning) methods will be considered for use to complete portions of MDOT projects provided a savings in schedule, time and costs, improved safety and reduced traffic control and costs, among other benefits can be shown while providing the required data accuracy to meet or exceed conventional surveying methods. The length/size of the project, GPS/GNSS collection environment, traffic volumes and available observation times are also considerations that affect the decision to use Mobile Terrestrial LiDAR.
  • Mobile Terrestrial LiDAR (MTL) generally consists of one or more LiDAR scanning units or sensors combined with multiple Global Navigation Satellite System (GNSS) receivers, an Inertial Measurement Unit (IMU), Distance Measuring Instrument (DMI) and multiple digital image or video cameras mounted on a moving terrestrial platform that collects highly accurate measurements and geospatial data.
  • LiDAR sensors use a moving projected light signal to measure the relative x, y, z, position and reflective properties of a point on an object. This results in a group of 3 dimensional points called a point cloud that can be colorized with information from the intensities or photo images to provide a product that is similar to other remote sensing products. To provide the point cloud in accurate real world coordinates that can be compared to previous surveyed positions, highly accurate ground surveys (that are tied to a known datum) are required. The geospatial data obtained with the collection of a point cloud can be extracted for a multitude of uses beyond the collection of topographic features required for typical corridor surveys. However, the point cloud data must be accompanied by a survey report describing the origin and accuracy of the data, for it to be used with confidence and to ensure the survey information, with any byproducts, are not misused.
  • The accuracy of individual points within point cloud data decreases as the distance from the LiDAR sensor increases. LiDAR sensor manufacturers provide specifications of precision for a given range. The overall range of the MTL system equipment will be dependent upon the acceptable accuracy required for the project, the precision of the LiDAR sensor, errors in GNSS measurement, errors in IMU measurements, errors in DMI measurement, errors in establishing horizontal and vertical control for the project and other similar error sources. Terrain, foliage, and other obstructions or partial obstructions may affect the point density/spacing and therefore the quality/accuracy of the resulting points in a portion of the point cloud. Care should be taken to ensure that the final point cloud(s) does not include any points with compromised accuracy caused by these conditions. Points collected beyond the limits of accuracy acceptable for the project must be filtered out and separated from the valid project data by classifying them as erroneous or invalid after processing.
  • Point density or spacing of collected data depends upon the measurement rate of the sensor and the speed of the sensor platform during measurement. The required point density for a project should be sufficiently close to be able to identify and extract physical detail to the accuracy specified for the project.
  • Refer to Table 1 on page 11 of the nchrp_rpt_748.pdf, Guidelines for the Use of Mobile LIDAR in Transportation Applications for general accuracy and point spacing requirements. The majority of MDOT Design Survey projects will likely fall in the 1A category requiring High accuracy and Fine point density.

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9.2.2 MTL Control and Geo-Referencing

  • In order to increase the accuracy of the collected MTL geospatial data, a local transformation of the point clouds must be conducted on MDOT projects, unless otherwise specified IN WRITING by the MDOT Survey Consultant Project Manager or MDOT Region Surveyor. There are many different types of local transformations that may be employed, however, the most common is a least squares adjustment of the horizontal and vertical residuals between established Local Transformation (adjustment) Points and the corresponding values from the point clouds to produce the transformation parameters of translation, rotation, and scale for the horizontal values and an inclined plane for the vertical values. These parameters are then applied to the point cloud to produce more accurate final geospatial data within the localized area of control.
  • Targets occupying known horizontal and vertical control incorporated in MTL surveys must serve as known local transformation points for point cloud adjustment and validation points for QA/QC. Targets must be of sufficient size and reflectivity to ensure redundancy of scan points sufficient for target identification and correct measurement within the point cloud.
  • The Local Transformation Points must be located at the beginning, end, and evenly spaced throughout the project to ensure that the project MTLS collection area is bracketed. The maximum distance with respect to route centerline stationing spacing between these points must be based on the type of survey. See diagram below.
  • Validation Points are used to check the geospatial data adjustment to the Local Transformation Points. Validation Points must be located at the beginning, end, and evenly spaced throughout the project. The maximum distance with respect to route centerline stationing spacing between these points must be based on the type of survey. See diagram below.
  • Variations to the spacing and layout scheme for Local Transformation Points and Validation Points are permitted to accommodate challenges or opportunities on projects, however any variation must be discussed and approved, in writing, by the MDOT Survey Consultant Project Manager or MDOT Region Surveyor.


Typical MTL Control and Validation Point Layout (CALTRANS Surveys Manual 2011) need to add image CH9.2.2_Typical_MTL_Point_Layout

Mobile Terrestrial LiDAR Equipment

  • MTL equipment and systems must be capable of providing data and results that meet MDOT engineering grade design requirements. More specific information in this section will be forthcoming.


Table 1 Mobile Terrestrial LiDAR Specifications

Mobile Terrestrial LiDAR Operation/Requirement Mobile Terrestrial LiDAR Application
MTL system equipment capable of collecting data at the intended accuracy and precision for the project. Required
Bore site calibration of MTL system when composed of multiple separate sub-units assembled on project site. Bore site per manufacturer’s specifications before and after project data collection. Required
Bore site calibration of MTL system when system is constructed as an integral package system with fixed orientation of component parts and a factory or company Bore Site prior to system use and no components have been changed. Optional but Suggested
Dual-frequency GNSS recording data at 1 Hz or faster Required
Inertial measurement unit (IMU) Required
Distance Measuring Instrument (DMI) Suggested

Specific Application of MTL on MDOT Projects

  • Any use of MTL technologies must conform to and utilize the information in this document and the following documents in the preparation of MDOT projects and deliverables:
    • Current MDOT Standards of Practice for Design Surveys
    • nchrp_rpt_748.pdf "Guidelines for the Use of Mobile LIDAR in Transportation Applications", April 2013
    • Caltrans Survey Manual Chapter 15 Terrestrial Laser Scanning Specifications
  • When Mobile Mapping is approved to be used on an MDOT project, the following must apply:
    • A Mobile Mapping trajectory plan and a ground control target layout plan (including the locations of any CORS GPS Stations and/or any local GPS base stations and how they are/were established) must be submitted with the work plan and priced proposal and prior to start of work for review by Lansing Design Survey Support and the MDOT Survey Project Manager.
      • LiDAR acquisition must be done when pavement is dry.
      • LiDAR acquisition should be obtained at traffic speeds to avoid impeding traffic but also ensuring adequate spacing from surrounding traffic so LiDAR collection is not obscured by traffic.
      • Multiple passes may be needed to eliminate as much obscured areas as possible.
      • LiDAR acquisition settings and operating speed must be done in a manner to maintain accurate data and consistent data spacing throughout the project. Data extracted from LiDAR must be delivered with a consistent spacing used by all operators. No significant spacing differences should be detectable in the extracted mapping throughout the project.
      • LiDAR acquisition must include collecting images along the trajectory routes to colorize the point cloud. Images will also be required for supplemental viewing by designers. These images must be rotated to an upright view, if needed, indexed, geo-referenced and delivered with associated trajectory and with the LiDAR data.
      • The trajectories, as driven, must be processed/refined, and, combined with the LiDAR data that has been acquired, must be registered to the ground control targets. A portion of the ground control targets (typically about 1/3 to 2/3 of the total targets) must be withheld from the registration process and used as independent points solely for validation of the point cloud and derived project data.

Interim Early Deliverables

  • Upon completion of the raw LiDAR processing and registration and prior to mapping extraction, copies of the following must be provided to Lansing Design Surveys and the Survey Project Manager for review:
    • Project report describing in detail how the mobile mapping project was done, number and general location of passes to acquire data, equipment used, datum surveyed on, and results of the processing.
    • The trajectory plan and a ground control target layout plan (including the locations of any CORS GPS Stations and/or any local GPS base stations) use to complete the LiDAR data acquisition. Provide a copy of manufacturer's trajectory plot, a set of TopoDOT TopoMission project files and set of .kml/.kmz files.
    • Reports and printouts from the processing software showing the results of the registration process.
    • Target to cloud registration - Statistics and comparison of the point cloud to the targets
    • Cloud to cloud registration - Statistics and comparison of adjoining overlapping point clouds.
    • Separation of forward and reverse solution (difference between forward and reverse post-process roll, pitch, yaw and XYZ positions solution).
    • Areas of the project that the data collected exceeded the maximum elapsed time or distance traveled of uncorrected IMU drift due to GNSS signal loss or obstruction.
    • Comparison of elevation data from overlapping (side lap) runs
    • Comparison of points at the area of overlap (end lap) if more than one GNSS base is used.
    • Primary control list and least squares adjustment reports (i.e. LGO, Starnet, etc.) for the control.
    • Intermediate control list and least squares adjustment reports (i.e. LGO, Starnet, etc.) for the control.
    • Point listing of the Targets used for the Mobile mapping and the least squares adjustment reports (i.e. LGO, Starnet, etc.) for these points.
    • Point listing of the Validation points used for the Mobile mapping and the least squares adjustment reports (i.e. LGO, Starnet, etc.) for these points.
    • Project Accuracy reports of IMU data.
    • Project Accuracy reports of GNSS data as well as PDOP and SV visibility.
    • Project Trajectory reports showing forward/reverse comparison and amount of difference between runs, final results of combined forward/reverse adjusted trajectory and report of accuracies to the project control.
    • Comparison spreadsheet showing the differences (fit) of the point cloud to the validation points. This spreadsheet must include a resultant summary in NSSDA format showing the 95% difference in horizontal X and Y, and Vertical Z for the project.
    • 3D Microstation DGN file and a .kmz file providing the graphical representation of the resulting differences between the project Point Cloud dataset and the targets and validation points.

Final Deliverables

  • Copies of the final deliverables must be provided to the Survey Project Manager and to Lansing Design Surveys and must include the following:
    • Project report describing in detail how the mobile mapping project was done, number and general location of passes to acquire data, equipment used, datum surveyed on, and results.
    • Complete listing of the Registration processing reports listed above.
    • Comparison spreadsheet showing the fit of the control points/targets to the point cloud. (The TopoDOT Control points to point cloud analysis tool that outputs a spreadsheet and chart diagram is one accepted option.)
    • Comparison spreadsheet showing the fit of the validation points/targets to the processed point cloud.
    • Comparison spreadsheet showing the differences (fit) of the final project DTM surface to the validation points. This spreadsheet must include a resultant summary in NSSDA format showing the 95% difference in horizontal X and Y, and Vertical Z for the project.
    • 3D Microstation DGN file and a .kmz file providing the graphical representation of the resulting differences between the final project DTM surface and the targets and validation points.
    • 3D Microstation DGN file containing all mapping extracted from LiDAR point cloud.
    • 3D Microstation DGN triangle file containing the terrain surface triangles created from the point cloud data.
    • Terrain surface saved as a Geopak .TIN file generated from the point cloud data.
    • LiDAR data tiled and saved in scanner native file format (such as Riegl .3dd, Cyclone .pts/.imp, etc.)
    • LiDAR data with RGB and Intensity values tiled and saved as colorized .POD (Point Tools/Microstation point cloud file) files.
    • DGN file showing the tile layout and naming of the .POD files. (If possible, use the same tile layout for both .POD and .LAS files.)
    • LiDAR data collected must be submitted in .LAS format with RGB values and intensity values.
    • DGN file showing the tile layout and naming of the .LAS files. (If possible, use the same tile layout for both .POD and .LAS files.)
    • Photo mosaic/Images along route that support the LiDAR .LAS point cloud. Provide a kml/kmz file, dgn index or direct folder naming that describes the organization of the images for easy access.
  • Point cloud files must be provided on electronic media of an appropriate size to contain all project information. Two (2) copies must be provided to MDOT Lansing Survey Support Unit. A third (3rd) copy must be provided for use by the MDOT Region Surveyor.
  • LiDAR and Mobile Mapping information should be prepared and placed in a separate sub-folder named "LiDAR/Mobile Mapping" under the Project Mapping Folder and must contain all information and LiDAR/Mobile Mapping deliverables relative to the project.
  • When other methods of survey and mapping are also employed for the project, the data must be combined and merged with the mobile mapping data and the merged information provided in the appropriate electronic files per the project's scope and requested deliverables.


References

  • The following references are good sources of information regarding the application of MTL. It is recommended these sources be referred to in preparing MDOT MTL projects. Where conflicts occur between these reference sources and MDOT documents, this Section (MTL Scanning Standards) and the MDOT scope take priority and the MDOT Survey Support Unit and MDOT Survey Project Manager should be contacted for clarification.
  • This Section was prepared based on experience from MTL projects done to date at MDOT and from adaptions of information from the following sources:

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