Anchor: #i1027461

Section 4: Photogrammetry

Anchor: #i1110016

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 Ground Control Submission Form (Form 2154). Contractors should request this form from the TxDOT District Survey Coordinator.

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

Anchor: #i1110650

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.
Anchor: #i1111715

Control Targets — Design

The following figure 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 Dimension. (click in image to see full-size image)

Figure 3-1. Target Design and Dimension.

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 3-2. 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.

Anchor: #i1113116

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

Anchor: #i1114888

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 3-4. 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 3.4 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

447

Vegetation

 

 

 

Woods

no

p_veg woods

448

Tree

no

p_veg tree

449

Tree Farm

no

p_veg tree farm

450

Tree Orchard

no

p_veg tree orchard

451

Palm

no

p_veg palm

452

Digital Terrain Model (DTM)

 

 

 

Breakline

yes

p_dtm breakline

453

General Breakline

yes

p_dtm general breakline

454

Retaining Wall Breakline

yes

p_dtm retaining wall

455

Sidewalk Breakline

yes

p_dtm sidewalk

456

Mass Points

yes

p_dtm mass points

457

Water Obscured

yes

p_dtm water obscured

458

Obscured Area

yes

p_dtm obscured area

459

Pit and Fill Area

yes

p_dtm pit or fill area

460

Stock Pile

yes

p_dtm stock pile

461



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

  • Aerial film negatives delivered on the file roll to the TxDOT District Survey Coordinator. The film will be archived at TSD.
  • Contact prints as required by the TxDOT District Survey Coordinator.
  • Control point documentation.
  • MicroStation® planimetric file as a 2D design file.
  • MicroStation® DTM data file as a 3D design file.
  • GEOPAK® TIN file.

If a field check (see subheading, “Accuracy Specification” below) is performed and additional deliverable of the statistical computation sheets is required.

II. Map Accuracy Standard

The TxDOT map accuracy standard for maps created using aerial photography is Class 1 of the American Society for Photogrammetry and Remote Sensing (ASPRS) Specifications and Standards Committee Standards for Large-Scale Maps (ASPRS 1990).Map accuracy depends on all of the individual components that go into the mapping process: ground control accuracy, precision, and spatial geometry; aerial photo acquisition, quality, and processing; analytical triangulation equipment and procedures; and map compilation equipment, procedures, and accuracy.

A mapping project can be designed with processes and procedures in place to meet a particular map accuracy specification. However, actual map accuracy can only be determined by an independent field check of the map using surveying equipment and procedures that result in higher accuracy coordinate determination than those of the map being checked.The TxDOT map accuracy standard allows production of photogrammetrically derived maps to adhere to the Class 1 ASPRS standard without the necessity of a field check provided the proper equipment and procedures are used. However, periodically TxDOT may request a field check on a map or may perform a field check using TxDOT equipment to ensure that the required accuracy standards are being met.

Anchor: #i1152454

Accuracy Specification

The standard places limits on the root mean square error (RMSE) for individual position components - northing, easting, and elevation.The limiting horizontal RMSE for large scale (1:20,000 or larger) Class 1 maps is 0.01'' at map scale. Therefore a map compiled to a scale of 1'' = 50' (1:600) would have a limiting RMSE for the X and Y components of 0.5'.

The following table gives the horizontal limiting RMSE for some common maps scales:

Anchor: #i1237619Table 3.5 Horizontal Limiting RMSE

Limiting RMSE Feet

Map Scale - Imperial

Map Scale Ratio

Practical Users

0.05

1'' = 5'

1:60

 

0.1

1'' = 10'

1:120

Typical Ground Methods

0.2

1'' = 20'

1:240

 

0.3

1'' = 30'

1:360

 

0.4

1'' = 40'

1:480

 

0.5

1'' = 50'

1:600

Typical Photogrammetric Methods

1.0

1'' = 100'

1:1,200

 

2.0

1'' = 200'

1:2,400

 



The limiting vertical RMSE for large scale (1:20,000 or larger) Class 1 map elevation data is 1/3 of the indicated contour interval. Spot elevations are restricted to an RMSE of 1/6 of the contour interval. A map compiled for a one foot contour interval would therefore have a limiting RMSE for elevation data of 0.33'. The limiting RMSE for spot elevations would be 0.17'.

Anchor: #i1153157

Limiting RMS Error

Limiting RMS error places a limit on the statistical value for a sampling of differences in X, Y, and Z coordinate values between features on the photogrammetrically compiled map and those identical features surveyed in the field using equipment with a higher level of positional accuracy.

Limiting RMS error should not be confused with the maximum allowable error for any individual coordinate component. It is possible for individual differences to exceed the limiting RMS error while the overall RMSE meets specification. Mathematically, the limiting RMSE is determined using the following equation:

where:D2 = d12 + d22 + d32 + ….. + dn2

d = discrepancy in the X, Y, or Z coordinate direction = xmap - xcheck

n = total number of points checked on the map in the X, Y, or Z coordinate direction

Anchor: #i1153638

Map Accuracy Testing

Tests for compliance of a map are optional but are recommended if the accuracy isn’t reported by the data provider or if the map accuracy is questionable. TxDOT may request a map accuracy test from a vendor for any photogrammetrically derived map. Alternatively, TxDOT staff may perform an accuracy check using state-owned resources. A written summary of the results of the field check is required if a vendor is asked to perform the map check.

The written summary will include the project SRN number, the roadway name being mapped, the county the project is in, and the date that the field check was performed. A brief summary of the field procedures and equipment used shall be included in the summary.

A table shall be included in the summary which shows the:

  • point number and description of the check point
  • X map coordinate, the X field surveyed coordinate
  • difference between the two coordinates
  • difference squared.

The same data should be included for the Y coordinate. A separate table with the same information should be prepared for the Z coordinate. At the bottom of the difference squared column, the average of the squares and the square root of the average shall be shown and labeled. The square root of the average of the squared differences is the limiting root mean square error (RMSE) value. This is the value that will be used to determine if the map meets the required accuracy specification.

Testing for horizontal accuracy compliance is done by comparing the planimetric (X and Y) map coordinates of well-defined points on the ground to the coordinates of the same points as determined by a horizontal check survey of a higher accuracy. The check survey shall be designed to achieve standard deviations equal to or less than one-third of the RMSE selected for the map.

Testing for vertical accuracy compliance shall be accomplished by comparing the elevations of well-defined points from the map to corresponding elevations determined by a survey of higher accuracy. For purposes of checking elevations, the map position of the ground point may be shifted in any direction by an amount equal to twice the limiting RMSE in position. The vertical check survey should be designed to produce RMSE in elevation differences at check point locations no larger than 1/10th of the indicated contour interval.

A minimum of 20 check points shall be used for field checking in order to produce acceptable statistical results. Check points should be evenly distributed throughout the entire map area. Discrepancies between X, Y, or Z coordinates of the ground point, as determined from the map and by the check survey that exceed three times the limiting RMSE shall be interpreted as blunders and will be corrected before the map is considered to meet this standard. All points determined to be blunders either on the map or as part of the field check survey shall be documented in the map check summary.

The following information shows typical statistical check sheets for horizontal and vertical accuracy of an aerial derived map.

Anchor: #i1285107Example of Horizontal & Vertical Map Check Statistical Computations

IH 45 Harris County, Houston District 12 12-76-0912-00-146-150

Field Check Date: 16 Apr. 2008

A

Point

#

B

Point

description

C

x

(independent)

D

x

(test)

E

(diff in x)

F

(diff in x)2

G

y

(independent)

H

y

(test)

I

diff in y

J

(diff in x)2

K

(diff in x)2 + (diff in y)2

1

top of conc pad

3099970.09

3099970.38

-0.29

0.0841

13909432.16

13909432.07

0.09

0.0081

0.0922

2

crn drop inlet

3103665.76

3103665.79

-0.03

0.0009

13910295.25

13910295.25

0

0

0.0009

3

crn sidewalk

3105015.41

3105015.38

0.03

0.0009

13910064.06

13910064.12

-0.06

0.0036

0.0045

4

crn drop inlet

3103214.54

3103214.81

-0.07

0.0049

13908108.33

13908108.31

0.02

0.0004

0.0053

5

crn drop inlet

3106206.65

3106206.63

0.02

0.0004

13906644.21

13906644.12

0.09

0.0081

0.0085

6

crn sidewalk

3104496.04

3104496.06

-0.02

0.0004

13905028.56

13905028.55

0.01

1E-04

0.0005

7

crn metal cover

3104600.95

3104601.14

-0.19

0.0361

13900398.39

13900398.28

0.11

0.0121

0.0482

8

crn drop inlet

3105032.84

3105032.88

-0.04

0.0016

13898292.88

13898292.67

0.21

0.0441

0.0457

9

crn drop inlet

3104704.35

3104704.53

-0.18

0.0324

13895887.81

13895887.69

0.12

0.0144

0.0468

10

crn sidewalk

3105204.89

3105204.96

-0.07

0.0049

13892376.37

13892376.31

0.06

0.0036

0.0085

11

crn driveway

3105809.82

3105809.75

0.06

0.0036

13887825.24

13887825.05

0.19

0.0361

0.0397

12

crn conc pad

3104858.29

3104858.37

-0.08

0.0064

13884163.09

13884163.04

0.05

0.0025

0.0089

13

top of conc pad

3106349.42

3106349.58

-0.14

0.0196

13882548.92

13882549.01

-0.09

0.0081

0.0277

14

crn drop inlet

3106855.43

3106855.29

0.14

0.0198

13879936.41

13879936.49

-0.08

0.0064

0.026

15

crn parking lot

3109355.47

3109355.34

0.13

0.0169

13876047.06

1387047.08

-0.02

0.0004

0.0173

16

crn drop inlet

3110849.61

3110849.70

-0.09

0.0081

13874828.43

13874828.17

0.26

0.0676

0.0757

17

crn sidewalk

3112408.46

3112408.60

-0.14

0.0196

13871487.38

13871487.52

-0.14

0.0196

0.0392

18

crn sidewalk

3114587.28

3114597.33

-0.05

0.0025

13869028.36

13869028.50

-0.14

0.0196

0.0221

19

crn sidewalk

3115583.99

3115584.58

-0.69

0.4761

13865356.61

13865356.73

-0.12

0.0144

0.4905

20

crn storm inlet

3117229.10

3117228.97

0.13

0.0169

1386315383

13863153.92

-0.09

0.0081

0.025

21

crn sidewalk

3119073.49

3119073.33

0.16

0.0256

13857719.27

13857719.11

0.16

0.0256

0.0512

22

crn storm inlet

3118945.00

3118945.00

0

0

13854865.18

13854865.24

-0.06

0.0036

0.0036

23

crn sidewalk

3119414.41

3119414.45

-0.04

0.0016

13852453.90

13852453.96

-0.06

0.0036

0.0052

24

crn sidewalk

31197.4639

3119746.29

0.1

0.01

13849397.99

13849398.19

-0.2

0.04

0.05

25

crn sidewalk

3121243.48

3121243.54

-0.06

0.0036

13843589.94

13843589.85

0.09

0.0081

0.0117

26

crn sidewalk

3118094.70

3118094.70

0

0

13841274.66

13841274.38

0.28

0.0784

0.0784

27

crn drop inlet

3118575.83

3118575.88

-0.05

0.0025

13838202.89

13838202.99

-0.1

0.01

0.0125

28

crn sidewalk

3122567.05

3122567.28

-0.23

0.0529

13836626.51

13836626.43

0.08

0.0064

0.0593

29*

crn storm inlet

 

 

 

 

 

 

 

 

 

30

crn storm inlet

3127609.06

3127609.12

-0.06

0.0036

13833599.74

13833599.86

-0.12

0.0144

0.018

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

sum

0.8557

 

 

sum

0.4674

 

 

 

 

 

average

0.0295069

 

 

average

0.016117

 

 

 

 

 

RMSE

0.17177572

 

 

RMSE

0.1269537

 

 

 

* Point not surveyed

Number of check points =29

ASPRS Class 1 limiting RMSE for horizontal accuracy is 0.01 inch at the map scale.

For maps compiled at a scale of 1'' = 50' the limiting RMSE is 0.5 feet

 

 

sum

1.3231

average

0.045624138

RMSE

0.213598076

NSSDA

0.369695549



Continuation of typical statistical check sheets for horizontal and vertical accuracy of an aerial derived map.

Anchor: #i1370378Example of Horizontal & Vertical Map Check Statistical Computations

IH 45 Harris County, Houston District 12 12-76-0912-00146-15

Field Check Date: 16 April 2008

A

Point #

B

Point description

C

z

(independent)

D

z

(test)

E

(diff in z)

F

(diff in z)2

1

top of conc pad

95.37

94.80

0.566

0.320356

2

crn drop inlet

88.64

88.39

0.245

0.060025

3

crn sidewalk

86.01

86.06

-0.05

0.0025

4

crn drop inlet

87.40

87.34

0.062

0.003844

5

crn drop inlet

83.84

83.68

0.159

0.025281

6

crn sidewalk

86.99

86.96

0.032

0.001024

7

crn metal cover

83.83

83.66

0.17

0.0289

8

crn drop inlet

80.70

80.37

0.331

0.109561

9

crn drop inlet

80.90

80.61

0.287

0.082369

10

crn sidewalk

76.39

76.25

0.137

0.018769

11

crn driveway

81.94

81.72

0.22

0.0484

12

crn conic pad

82.96

82.84

0.115

0.013225

13

top of conc pad

81.46

81.16

0.303

0.091809

14

crn drop inlet

81.32

81.04

0.279

0.077841

15

crn parking lot

73.58

73.21

0.368

0.135424

16

crn drop inlet

70.45

70.04

0.405

0.164025

17

crn sidewalk

65.06

64.49

0.572

0.327184

18

crn sidewalk

63.09

62.70

0.393

0.154449

19

crn driveway

55.34

55.16

0.176

0.030976

20

crn storm inlet

58.96

58.65

0.31

0.0961

21

crn sidewalk

50.67

50.40

0.265

0.070225

22

crn storm inlet

46.47

45.90

0.567

0.321489

23

crn sidewalk

51.42

51.19

0.234

0.054756

24

crn sidewalk

27.12

26.61

0.514

0.264196

25

crn sidewalk

43.26

42.88

0.379

0.143641

26

crn sidewalk

45.45

45.07

0.376

0.141376

27

crn drop inlet

46.02

45.94

0.084

0.007056

28

crn sidewalk

43.72

43.40

0.316

0.099856

29*

crn storm inlet

 

 

 

 

30

crn storm inlet

36.97

37.50

-0.535

0.286225

 

 

sum

3.180882

*Point not surveyed

average

0.10968559

Number of check points = 29

RMSE

0.33118814

 

NSSDA

0.64912876

ASPS Class 1 limiting RMMSE value for elevation is 1/3 of indicated contour interval. Spot elevation RMSE is 1/6 of the indicated contour interval. For 1 foot contour mapping the limiting RMSE is 0.33 feet and 0.17 feet for spot elevations.



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Statement of Map Accuracy

For maps that are not field checked but have been compiled to meet TxDOT’s accuracy specification, the following statement shall be included on the delivered hardcopy and digital versions of the map:

THIS MAP WAS COMPILED TO MEET THE ASPRS STANDARD FOR CLASS 1 MAP ACCURACY

If the map was checked and found to conform to this spatial accuracy standard, both the statement above and the following statement shall be included on the delivered hardcopy and digital versions of the map, as well as in the field check summary:

THIS MAP WAS CHECKED AND FOUND TO CONFORM TO THE ASPRS STANDARD FOR CLASS 1 MAP ACCURACY

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Step-by-Step Accuracy Testing

  1. Select the check points. Select a minimum of 20 well-defined points in the map dataset. A well-defined point represents a feature for which the horizontal position is known to a high level of accuracy. For the purpose of accuracy testing, well-defined points must be clearly defined on the map product and easily recoverable on the ground. Check points shall be evenly distributed throughout the area covered by the map.
  2. Field survey the check points. Using a survey method of higher accuracy (typical optical or GPS surveying equipment) occupy the selected check points and record the X, Y, and/or Z value for each point. Note that a single point can be used to check both planimetric and elevation accuracy. The survey method should be capable of measuring with a horizontal accuracy one-third the selected limiting RMSE value for the map. The survey should also be capable of a vertical measurement 1/10th of the indicated contour interval of the map.
  3. Document the horizontal test. Produce a table listing: Point number, description, X coordinate determined from the source of higher accuracy, X coordinate determined from the map, the difference between the two coordinates, the squared difference, Y from the source, Y from the map, the difference, the squared difference. This table will be included in the field check summary documentation.
    1. Sum the squared differences and compute the average. From the table column containing the X squared difference, sum the values and compute the average difference. Do the same for the column containing the Y squared difference values.
    2. Calculate the root mean squared error. Use the root-mean-square error (RMSE) to determine the positional accuracy in both the X and Y coordinate direction. RMSE is the square root of the average of the set of squared differences between dataset coordinate values and coordinate values from an independent source of higher accuracy for the identical points. Take the square root of the average computed in the previous step for both the X and Y columns. This value is the limiting RMSE for the map. This value should not exceed the limiting RMSE specification for the map scale being checked. If the computed RMSE exceeds the limiting RMSE value for the map scale, the map fails its accuracy specification. The equation for the RMSE test is as follows:

    where:

    D2 = d12 + d22 + d32 + ….. + dn 2

    d = discrepancy in the X or Y coordinate direction = Xmap - Xcheck

    n = total number of points checked on the map in the X or Y coordinate direction

  4. Document the vertical test. Produce a table listing the: Point number, description, Z coordinate determined from the source of higher accuracy, Z coordinate determined from the map, the difference between the two coordinates, and the squared difference. This table will be included in the field check summary documentation.
    1. Sum the squared differences and compute the average. From the table column containing the Z squared difference, sum the values and compute the average difference.
    2. Calculate the root mean squared error. Use the root-mean-square error (RMSE) to determine the positional accuracy in Z coordinate direction. RMSE is the square root of the average of the set of squared differences between dataset coordinate values and coordinate values from an independent source of higher accuracy for the identical points. Take the square root of the average computed in the previous step for the Z column. This value is the limiting RMSE for the map. This value should not exceed the limiting RMSE specification for the map scale being checked. If the computed RMSE exceeds the limiting RMSE value for the map scale, the map fails its accuracy specification. The equation for the RMSE test follows:

    where:

    D2 = d12 + d22 + d32 + ….. + dn 2

    d = discrepancy in the Z coordinate direction = Zmap - Zcheck

    n = total number of points checked on the map in the Z coordinate direction

  5. Prepare the summary. The written summary will include the project SRN number, the name of the roadway being mapped, the county that the project is in, and the date that the field check was performed. A brief summary of the field procedures and equipment used should be included. The tables prepared from the data listed above shall be included in the summary. The table will include the point number and description of the check point, the X map coordinate, the X field coordinate, the difference between the two points, and the difference squared. The same data should be included for the Y coordinate. The table prepared from the data listed above for the Z coordinate check shall also be included in the summary. At the bottom of the difference squared column, the average of the squares and the square root of the average should be shown and labeled. The square root of the average of the squared differences is the RMSE value.
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