Chapter 4 - Photogrammetry

Survey ManualChapter 4Aerial SurveysColorado Department of TransportationDecember 30, 2015

TABLE OF CONTENTSChapter 4 – Aerial Surveys4.1 General . 44.1.1 Acronyms found in this Chapter . 44.1.2 Purpose of this Chapter . 54.1.3 Aerial Surveys . 54.1.4 Aerial Photogrammetry . 54.1.5 Photogrammetric Advantages / Disadvantages . 64.1.6 Aerial LiDAR . 64.1.7 LiDAR Advantages / Disadvantages. 74.1.8 Pre-survey Conference – Aerial Survey . 94.2 Ground Control for Aerial Surveys . 104.2.1 General . 104.2.2 Ground Control Targeting Requirements . 104.2.3 Photogrammetry . 114.2.4 Aerial LiDAR . 124.2.5 Equipment Checking and Calibration . 124.2.6 Permission to Enter Property Form 730a . 124.2.7 Underground Utility Locates Prior to Installing Photo Control Monumentation . 134.2.8 Aerial Ground Control Monumentation . 134.2.9 Center Point Control. 134.2.10 Wing Point Control . 144.2.11 Aerial Control Targets (Paneling) . 144.2.11.14.2.11.2Photogrammetry . 14LiDAR . 154.2.12 Aerial Control Target Design & Material . 154.2.13 Removal of Aerial Control Target Material . 164.3 Aerial Control Horizontal Survey . 174.3.1 Aerial Control Horizontal Survey Datum . 174.3.2 Minimum Aerial Control Horizontal Survey Accuracy Tolerance . 174.3.3 GPS Photo Control Horizontal Survey Methods. 174.3.4 Conventional Aerial Control Horizontal Survey Methods . 184.4 Aerial Control Vertical Survey . 194.4.1 Photo Control Vertical Survey Datum (NAVD 88) . 194.4.2 Minimum Aerial Control Vertical Accuracy Tolerance . 194.4.3 GPS Aerial Control Vertical Survey Methods . 194.4.4 Conventional Aerial Control Vertical Survey Methods . 194.5 Aerial Control Survey Report. 204.5.1 General . 204.5.2 Aerial Control Survey Report . 204.6 Aerial Topo Mapping Standards . 224.6.1 CDOT CADD Standards . 224.6.2 MicroStation/InRoads Configurations for Consultants . 224.6.3 MicroStation Level Structure . 224.6.4 Aerial Survey – Photogrammetric Feature Identification . 22CDOT Survey Manual2December 30, 2015

4.6.54.6.6Post Aerial or Pre-Aerial TMOSS Supplemental Surveys . 22Minimum Horizontal and Vertical Accuracy Tolerance for TMOSS Supplemental Survey . 234.7 Aerial Mapping Tolerances . 244.7.1 Aerial Mapping Horizontal Accuracy Tolerance . 244.7.1.14.7.24.7.34.7.44.7.5Orthophotography . 24Aerial Mapping Vertical Accuracy Tolerance . 25Existing Constructed Transportation Corridor Template . 25Obscured Areas . 25Vertical Accuracy Testing - . 25Method of Verifying Accuracy Tolerance . 254.7.5.14.7.5.2Photogrammetry . 25Aerial LiDAR . 264.8 Aerial Surveys and Photogrammetry Specifications . 274.8.1 General . 274.8.2 American Society for Photogrammetry & Remote Sensing (ASPRS) . 274.8.3 Project Location and Limits . 274.8.4 Aerial Survey Field Conditions . 284.8.5 Flight Plan . 284.8.6 Aircraft . 294.8.7 Aerial Data Acquisition . 294.8.8 Raw Data . 74.8.8.84.8.8.94.8.8.10Imagery Quality . 29Film Labeling. 30Aerial Triangulation. 30Digital Image Naming Convention . 31Aerial LiDAR - General . 31Aerial LiDAR Data Application . 32LiDAR Data Calibration Results . 32LiDAR Point Cloud . 33LiDAR Tile Layout, File Naming . 33Re-flights . 344.9 Deliverables . 354.9.1 General . 354.9.2 Photo Index . 354.9.3 Planimetric Features . 36Planimetric Maps - . 364.9.4 Nomenclature . 364.9.5 Digital Terrain Models (DTM) . ital Terrain Models from LiDAR . 36Triangular Irregular Network (TIN) . 36Contours . 37Orthophotography (Orthophoto) . 37Aerial Survey Report . 38Deliverable File Naming . 39Raw Data Files . 39References . 40CDOT Survey Manual3December 30, 2015

SNSRSNSSDANVAPDOPPIDPPRRGBRMSEVVAAcronyms found in this ChapterAirborne Global Positioning System (See also the application of GPS in this document.)American Standard Code for Information ExchangeAbove Ground LevelAmerican Society for Photogrammetry and Remote SensingAir Traffic ControlColorado Department of TransportationDigital Elevation ModelThis is a reference to a Bentley MicroStation file format. The term is based on the file nameextension used for this format.Digital Terrain ModelFederal Aviation AdministrationForward Motion CompensationGeographic Information SystemGlobal Navigation Satellite System (See also the application of GPS in this document.)Global Positioning System (In this document it refers generically to space satellite systempositioning. See Chapter 3 for application of specific space satellite systems.)Ground Sampling Distancehertz (frequency per second)Inertial Navigation SystemInertial Measurement UnitThis is a reference to an industry standard LiDAR point cloud file format. The term is basedon the file name extension used for this format.Light Detection and RangingManual of Uniform Traffic Control DevicesNorth American DatumNorth American Vertical DatumNear Infrared: Used in reference to a part of the light spectrum.National Geodetic SurveyNational Spatial Reference SystemNational Standard for Spatial Data AccuracyNon-vegetated Vertical AccuracyPositional Dilution of PositionPhoto ID, used in reference to aerial mapping ground control points naturally identifiable inaerial imagery without the need to target, (or panel) the control point.Prior Permission RequestRed, Green, Blue, used in reference to parts of the visible light spectrum.Root Mean Square Error (may be followed by lower-case “z” as referred to elevation or an “r”if referencing a radial distance - x & y combined)Vegetated Vertical AccuracyCDOT Survey Manual4December 30, 2015

4.1.2Purpose of this ChapterThe purpose of this chapter is to define the specifications that shall be followed while performing aerialsurveys, photogrammetry and geospatial data processing for CDOT.CDOT contracts out all aerial surveys as the aerial photography and mapping equipment is not available inthe department. As such CDOT relies upon the expertise and experience of the aerial mapping consultantto provide guidance and products that will meet the needs of the project. The survey fieldwork is mostoften performed by the aerial consultant however it may also be performed by CDOT survey crews.The guidelines and specifications described in this chapter are geared towards development of design scalemapping that has been historically referred to as 1” 50’ scale mapping with 1’ contours. The vast majorityof aerial mapping contracted by CDOT calls for mapping standards associated with this scale. Whererequirements differ from this scale, the necessary equipment, ground control, flight planning and other keycomponents of the project design may need to be modified. This may be accomplished either to ensure ahigher standard is met or to realize efficiencies that may be offered to meet a lower standard. Anyvariation from the specifications in this chapter shall have the prior approval of the CDOT Region SurveyCoordinator.While it is recognized that technical developments, particularly in airborne LiDAR, are making widerapplication of aerial data possible for design scale mapping, this chapter provides specifications andguidelines for LiDAR data used alone or in conjunction with photogrammetry supplementing field surveydata on the hard road surfaces. Certain circumstances may call for consideration of wider data applicationsuch as full detail extraction from a high-density LiDAR point cloud. Where accessibility, safety,economics or other concerns call for such consideration it should be done in consultation between aprofessional aerial surveyor, such as an ASPRS Certified Photogrammetrist, map scientist or state licensedaerial survey professional and the CDOT Region Survey Coordinator. This will facilitate development ofa custom project design, specifications, and deliverables that meet unique CDOT project requirements.Again, any variation from the specifications in this chapter shall have the prior approval of the CDOTRegion Survey Coordinator.4.1.3Aerial SurveysAerial surveys utilize photographic, LiDAR, electronic, digital, or other data obtained from an airborneplatform. Photographic data processed by means of photogrammetry and LiDAR processing using AGPSand IMU data represent the principal applications of aerial surveys to satisfy the needs of CDOT. Aerialsurvey data is combined with field survey data to produce high precision mapping and meet the accuracystandards described in this Chapter.4.1.4Aerial PhotogrammetryAerial photogrammetry is the science of deducing the physical dimensions of objects on or above thesurface of the Earth from measurements on aerial photographs of the objects. The end result produces thecoordinate (X, Y, and Z) position of a particular point, a planimetric feature, and a graphic representationof the terrain from a DTM.Aerial photogrammetry is often used for the following:1.2.3.4.Highway reconnaissanceEnvironmentalPreliminary designGeographic Information System (GIS)The information produced from aerial photographs of the existing terrain allows both designers andenvironmental personnel to explore alternate routes without having to collect additional field information.CDOT Survey Manual5December 30, 2015

The photographs can be used to layout possible alignments for a more detailed study.Photogrammetry has evolved into a limited substitution for topographic ground surveying. It can relievesurvey crews of the most tedious time-consuming tasks required to produce topographic maps and DTMs.However, ground surveys will always remain an indispensable part of aerial surveys as a basis foraccuracy refinement, quality control and a source of supplemental information unavailable to aerial dataacquisition.4.1.5Photogrammetric Advantages / DisadvantagesSurveys collected by aerial photogrammetry methods have both advantages and disadvantages whencompared with ground survey methods as follows:Advantages:1. Photos provide a permanent record of the existing terrain conditions at the time the photograph wastaken.2. Photos can be used to convey information to the general public, and other federal, state, or localagencies.3. Photos can be used for multiple purposes within CDOT such as reconnaissance, preliminary design,environmental, and Right of Way.4. Topographic mapping and DTMs of large areas can be accomplished relatively quickly and at alower cost when compared to ground survey methods.5. Photogrammetry can be used in locations that are difficult or impossible to access from the ground.Disadvantages:1. Seasonal conditions, including weather, vegetation, and shadows can affect both the taking ofphotographs and the resulting measurement quality. If the ground is not visible in the photograph itcannot be mapped.2. Overall accuracy is relative to camera quality and flying height. Elevations derived fromphotogrammetry are less accurate than ground surveys (when compared to conventional or GPSground survey methods using appropriate ele

4.1.4 Aerial Photogrammetry Aerial photogrammetry is the science of deducing the physical dimensions of objects on or above the surface of the Earth from measurements on aerial photographs of the objects. The end result produces the coordinate (X, Y, and Z) position of a particular point, a planimetric feature, and a graphic representation