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Section 4: Photogrammetry

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Control Target Documentation

All control points will be documented with:

  1. a graphical representation of the location of markers
  2. a textual documentation in the form of a spreadsheet.

The above information shall be provided to the TxDOT District Survey Coordinator. However, the TxDOT District Survey Coordinator may require additional documentation depending on the requirements of the project.

Graphical documentation presented as a sketch or CAD drawing shall include the following information.

  • Graphical information:
  • spatial location of control point with sufficient detail to allow recovery of the point in the field
  • map scale and north arrow
  • stationing annotated project centerline or roadway alignment (if available) as supplied by TxDOT.
  • Textual information:
  • date of installation
  • highway name and project limits
  • general description of the location of point referenced to highway intersections, city limits, etc.
  • TxDOT work order number and control section job (CSJ) number (if applicable)
  • identification of reference coordinate system and elevation datum
  • combined surface adjustment (CAF) or TxDOT Surface Adjustment Factor (SAF)
  • point coordinates identified as being either grid or surface coordinates.

Textual documentation shall be submitted using the Microsoft Excel spreadsheet titled Ground Control Submission Form (2154). See Figure 3-1 below for an example of the form.

Note that the standard deviations computed from the network adjustment shall be included on this form.

Ground Control Submission Form (2154). (click in image to see full-size image)

Figure 3-1. Ground Control Submission Form (2154).

(click in image to see full-size image)

Second page of Form 2154. The above form may be access through the internal TxDOT Forms System. Contractors should request this form from the TxDOT District Survey Coordinator.

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Control Targets - General

The secondary control network described in the previous section is marked with cross-shaped targets during the aerial photography flight. The targets are visible on the developed aerial film and are used to relate the aerial photography to the ground. Targeting is an essential part of photogrammetric mapping. Studies conducted by the Federal Highway Administration (FHWA) determined that pre-marked targets improve the efficiency of photogrammetric processing and increase overall accuracy of point measurements.

A right-of-entry agreement with a landowner must be obtained prior to entering private property to set photogrammetric control. The agreement shall include detail on the length of time that the control target will be on the ground and those responsible for removal of the material. The agreement may include additional detail such as specific times for access to the property or conditions on notification before entering the property.

Targets may be placed on hard surfaces such as pavement or concrete or on soft surfaces such as soil, gravel, or grass. In general, hard surfaces are preferred because point measurements both for the surveyor and for the photogrammetrist, are more accurate on a hard surface. In addition, a hard surface allows targets to be painted which increases the durability and longevity of the mark.

The cross-shaped target is centered on the PK nail, iron rod, or other surveying marker that defines the control point. On hard surfaces the targets shall be painted using flat finish paint. The target color shall be either white (on darker background surfaces) or black (on lighter background surfaces). Contrast between the target and the background material is important for point measurement in the photogrammetric process. It is permissible to use both paint colors, one as a background color, and the other as the target color to enhance the contrast further. However, in general, white targets are preferred.

On soft surfaces the target is constructed of cardboard, sheetrock, plastic film, Tyvek, or other similar materials durable enough to remain in place until the flight mission is complete and the film has been inspected. Once the flight mission has been approved, the target materials shall be picked up.

Placement of control targets shall meet the following criteria:

  • The target should be clear of any obstruction that may obscure the target on the aerial photography. When standing on the target, there should be a clear view of the sky from 45o above the horizon to zenith in every direction.
  • The target shall be placed as flush to the ground as possible. Any vegetation that may grow beneath the target should be cleared prior to placing the target material. Vegetation growth can cause a target to bow, affecting the accuracy of the photogrammetric measurement.
  • Targets should not be placed in a shadow or in a shady area. It is advisable to visit target locations at the approximate time of the flight mission to determine the location of shadows.
  • Placement of targets beneath overhead wires should be avoided. Overhead wires make measurement of the targets in the photogrammetric process problematic.
  • Targets should be placed on as level an area as possible. If a target must be placed on a slope, the target should be oriented so that two of the legs that form a straight line traverse the slope at approximately the same elevation.
  • The horizontal coordinate provided for the target shall be at the center of the target at the location of the control monument. The vertical elevation of the target shall be at the center of the target at the level of the target material. This is important because the photogrammetric measurement of the point is made at the elevation of the target material, which may or may not be the elevation of the survey marker.

Control Targets - Design

The following figure (Figure 3-2) shows the design of a typical control target. Note that the size of the target is dependent on the scale of the aerial photography for which it will be used. Target design and dimensions.

Target Design and Dimensions. (click in image to see full-size image)

Figure 3-2. Target Design and Dimensions.

For a typical 1'' = 250' photo scale the dimensions of the target are: Thickness (T) = 6 inches and Length (L) = 84 inches. For projects with different photo scales, consult with the photogrammetrist for the correct target dimensions.

The illustration below shows the design of a box type target used to designate new flight tangents.

Box Target Design. (click in image to see full-size image)

Figure 3-3. Box Target Design.

A flight tangent is a single, straight flight line maintained by the aircraft. Anytime the aircraft has to make a turn to maintain photo coverage over a roadway, that turn and new flight direction constitutes a new flight tangent. Generally, a point of intersection (PI) in a roadway of 15 degrees or more will require a new flight tangent.

Control Targets—Placement

Placement of the secondary control network survey markers and targets shall adhere to the following requirements:

  • A band of control is placed 300 to 500 feet before the beginning and ending control bands for the project. A band of control is defined as being the centerline survey marker and its associated wing markers.
  • Control bands are placed at 1500' maximum and 1000' minimum intervals along the flight line. The maximum distance between bands can be increased to 1800 feet but not for two or more consecutive bands.
  • Wing markers are set at the width of the mapping area with a maximum distance of 600 feet from the centerline. The minimum distance is typically 200 feet from the centerline marker. However, in situations where access to private property is denied or physical features prevent placement at the minimal of 200 feet, target can be placed at the edge of the right-of-way.
  • The first and last centerline survey point of each separate flight line shall be constructed as a “box” marker. Note that if the roadway being covered by the aerial photo mission has a PI greater than 15 degrees, a new separate flight line will be required to maintain photo coverage. In this case the new flight line will require box panels at the beginning and ending of the flight line.
  • All flight lines require a minimum three (3) horizontal markers.

The figure below illustrates the control marker and target layout for a typical aerial mapping project. Note the PI that occurs towards the center of the figure and the corresponding control point locations.

Example of Control Target Layout and Naming. (click in image to see full-size image)

Figure 3-4. Example of Control Target Layout and Naming.

Photogrammetric Mapping

I. Aerial Photography - General

Film titling should consistently be on the left side of the image frame regardless of the flight direction. The title will include the aerial photo date, the project SRN number (or the CSJ number or project number as directed by the TxDOT District Survey Coordinator), the highway or project name, and the photo strip and frame number. An example of film titling is provided below:

Example of Film Titling. (click in image to see full-size image)

Figure 3-5. Example of Film Titling.

All aerial film is the property of TxDOT. Film shall be delivered to the TxDOT District Survey Coordinator as a project deliverable. The film will be archived at the TxDOT Technology Services Division (TSD).

II. Aerial Mapping - General

Equipment used in the aerial mapping process is required to produce maps conforming to American Society of Photogrammetry and Remote Sensing (ASPRS) Class 1 Accuracy Standard. This requirement affects all equipment and workflows used in the mapping process including: aircraft operation, the aerial mapping camera, the film scanner, and the photogrammetric workstation. This requirement also affects operator proficiency for all systems and workflows involved in map production.

TxDOT requires that maps produced photogrammetrically be delivered in MicroStation ® V8 file format. The following level structure is required for the maps.

The following items are the standard deliverables for an aerial mapping project:

Anchor: #i1142604Table 3.5 Photogrammetric Level Structure

Photogrammetry Feature

DTM

Microstation V8 Name

Level

Control

 

 

 

Horizontal Control, Principal Point

no

p_control ground ctrl

401

Road

 

 

 

Paved Road, Curb

yes

p_road paved, curb

402

Dirt Road

yes

p_road dirt

403

Guard Rails

no

p_road guard rail

404

Guard Fences

no

p_road guard fence

405

Guard Posts

no

p_road guard post

406

Concrete Barrier

no

p_road conc barrier

407

Paint Stripe Solid and Dashed

yes

p_road paint stripe

408

Bridge End

yes

p_road bridge end

409

Cattle Guard

no

p_road cattle guard

410

Overhead Sign

no

p_road overhead sign

411

General Road Feature

no

p_road general feature

412

Railroad

 

 

 

Railroad Track RR Controls

no

p_railroad rr control

413

Drainage

 

 

 

Concrete Dam

yes

p_drainage conc dam

414

Concrete Drain

yes

p_drainage conc drain

415

Earthen Dam

yes

p_drainage earthen dam

416

Riprap

yes

p_drainage riprap

417

Culvert

yes

p_drainage culvert

418

Inlet

yes

p_drainage inlet

419

Water

yes

p_drainage water

420

Marsh

yes

p_drainage marsh

421

Structure

 

 

 

Building

no

p_structure building

422

Ruin

no

p_structure ruins

423

Sidewalk

no

p_structure sidewalk

424

Slab

no

p_structure slab

425

Porch, Deck

no

p_structure porch

426

Stairs, Steps

no

p_structure stairs

427

Fence, Gate, Post

no

p_structure fence

428

Retaining Wall

no

p_structure ret wall

429

Wall

no

p_structure wall

430

Cemetery

no

p_structure cemetery

431

Billboard

no

p_structure billboard

432

Sign, Sign Pole, Sign Post

no

p_structure sign

433

Antenna, Cellular Tower, Satellite Dish

no

p_structure antenna

434

Windmill

no

p_structure windmill

435

Flag Pole

no

p_structure flag pole

436

Pipes

no

p_structure pipe

437

Tank

no

p_structure tank

438

Area Under Construction

no

p_structure constr area

439

General, Ac Unit, Goal, Large, Small Circle

no

p_structure general

440

Unidentified Feature

no

p_structure unidentified

441

Utility

 

 

 

Fire Hydrant

no

p_utility fire hydrant

442

Manhole

no

p_utility manhole

443

Marker, Meter, Valve

no

p_utility marker

444

Transmission Tower, transmission Line

no

p_utility trans tower

445

Pipeline

no

p_utitlity pipeline

446

General, Pole, Pole LP, TFP, LP

 

 

 

Traffic Light, Gas Light

no

p_utility general pole