LiDAR DataAdvantages, Disadvantages And Getting What You Want

Steve Kasten CP, PLSstevek@surdex.com

Outline

■ Accuracy

■ Specifications

■ Comparisons to Photogrammetry and cost considerations

■ Data storage, tiling and software considerations

Accuracy

■ Combination of two parametersPositional accuracy of discrete points in

digital elevation modelHow well the data fits the form of the earth’s

surface

Accuracy

Precise, but not accurate Not Precise, not accurate

Precise, accurate

Utilizes well defined independent checkpoints (differences between higher order surveyed coordinates and product test coordinates are calculated for each checkpoint component)

Descriptive Statistics

Normal Distribution Function - A mathematical function describing the behavior of one-dimensional random errors whose graph is a bell-

shaped curve that extends indefinitely in both directions.

Error Statistics

1 sigma (1σ) level equals 68.26% of area under the curve 2 sigma (2σ) level equals 95.44% of area under the curve 3 sigma (3σ) level equals 99.74% of area under the curve

Confidence Level and the Bell Shaped Curve

Accuracy Standards National Map Accuracy Standards (1947)

USDOT Reference Guide Outline - Specifications for Aerial Surveys and Mapping by Photogrammetric Methods for Highways (1968)

ASPRS Accuracy Standards for Large-Scale Maps (1990)

National Standard for Spatial Data Accuracy (NSSDA) (1998)

National Digital Elevation Program (NDEP) (2004)

Dependent on Map Scale and Contour Interval

Accuracy Reporting “This map complies with National Map accuracy standards.”

Accuracy Requirements at 90% confidence level

Sets Pass/Fail limits of product

Does not define testing method

Horizontal Accuracy 90% of planimetric features are within 1/30” at map scale for map scales larger than 1:20,000

Vertical Accuracy 90% of elevations determined from contours are within 1/2 contour interval

National Map Accuracy Standards (1947)

Dependent on Map Scale and Contour Interval

Accuracy Reporting language not explicitly defined

Accuracy Requirements at 90% confidence level

Sets Pass/Fail limits of product

Does not define test method

Horizontal Accuracy

90% of planimetric features are within 1/40” at map scale

Vertical Accuracy

90% of elevations determined from contours are within 1/2 contour interval

90% of spot elevations are within 1/4 contour interval

USDOT Reference Guide Outline(1968)

Dependent on Map Scale and Contour Interval

Accuracy Reporting RMSE Class I, II, III

1/3 CI and 1/6 CI spots

Sets Pass/Fail limits of product

Defines testing method Accuracy Reporting (Testing)

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

Accuracy Reporting (Procedure)

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

ASPRS Accuracy Standards for Large-Scale Maps (1990)

Map Product Accuracy Horizontal Map Accuracy

0

2

4

6

8

10

12

14

20 40 50 100 200 400

Map Scale in Inch per Foot

RM

SE

in F

ee

t I

II

III

NMAS

Vertical Map Accuracy

0

1

2

3

4

5

6

0.5 1 2 4 5

Contour Interval in Feet

RM

SE

in F

ee

tI

II

III

NMAS

CI I II III NMAS

0.5 0.17 0.33 0.5 0.15

1 0.33 0.66 1 0.3

2 0.67 1.33 2 0.61

4 1.33 2.67 4 1.22

5 1.67 3.33 5 1.52

Scale I II III NMAS

20 0.2 0.4 0.6 0.31

40 0.4 0.8 1.2 0.62

50 0.5 1 1.5 0.78

100 1 2 3 1.55

200 2 4 6 3.11

400 4 8 12 6.21

Table values are RMSE in feet

Independent of Map Scale and Contour Interval

Accuracy Reporting at 95% confidence level and is based on either Testing or Procedure

Does not set Pass/Fail limits, leaves it to the user to determine

Defines testing method

Accuracy Reporting (Testing) Tested ____ (meters, feet) horizontal accuracy at 95% confidence levelTested ____ (meters, feet) vertical accuracy at 95% confidence level

Accuracy Reporting (Procedure)

Compiled to meet ___ (meters, feet) horizontal accuracy at 95% confidence levelCompiled to meet ___(meters, feet) vertical accuracy at 95% confidence level

NSSDA (1998)

NDEP (2004)■ Extension of NSSDA

Same statistical level■ Specific to elevation data■ Defines testing method

20 (30 preferred) points per vegetation typeLocated around features of interestLand cover types in the area of interest

―Open terrain―Tall weeds/crops―Brush lands and low trees―Forests―Urban

Project Example

Check Points

Asphalt Gravel Concrete

Grass Tall Grass Trees

Data Analysis

Summary

Linn and Sullivan

0.24

0.150.13

0.16

0.12

0.17

0.00

0.05

0.10

0.15

0.20

0.25

RMSE in

meters

Trees Grass Tall grass Gravel Asphalt Overall

Ground Type

Linn County Elevation Data Error

Topographic Data of Kansas

DEM Horizontal Resolution

Grid vs. Mass Points & Breaklines

Ground Surface Mass Points

Interpolated Surface

Ground Surface

Mass Points

Breaklines

Interpolated Surface

1-arc-second (30 meters) 1/3-arc-second (10 meters) 1/9-arc-second (3 meters)

How do you define your specifications?

Horizontal Standards

Vertical Standards

Data Density?

Elevation Data Sample Distance

FEMA Specifications

Comparisons to Photogrammetry

■ Photogrammetric Process

Define Corridor

Film

Place Photo Control

Scan

Digital Image

Aerial Triangulation

Generate Breaklines

DTM

Bare-Earth DEM

CreateOrthophoto

Contours TIN

Select Final Corridor

Field Survey

Densified DTM

Construction Plans

■ Combined LiDAR and Photogrammetric Process

Define Corridor

Place Photo Control GPS Control

Film Signal Returns

LIDAR Flight

LIDAR ProcessingScan

Digital Image

Aerial Triangulation

Generate Breaklines Bare-Earth DEM

Planning Level DTM

Create Ortho Contours TIN

Select Final Corridor

Additional Photo Controlfor Narrow Corridor

Aerial Triangulation

Generate AdditionalBreaklines

Densify Bare-Earth DTM

Local DTM

Construction Plans

Estimated Time and Cost Savings■ Highway Projects (Iowa DOT)■ Time

Photogrammetric mapping – estimated two years to produce

LIDAR – five months (addt’l. photogrammetry work, eight months)

Result – eleven months time savings■ Financial

Photogrammetry – est. $500,000LIDAR – est. $150,000 (addt’l photogrammetry $100,000)Result - $250,000 savings (50%) over photogrammetry

Estimated Time and Cost Savings■ Time

Photogrammetric mapping required 2,670 hours

LIDAR required 598 hoursSavings of 2,072 hours (71%) not

including time for final design

Contour CostsPhotogrammerty vs LIDAR

Dewberry and Davis

MNDOT LiDAR Study

File Size Considerations

File Formats■ ASCII Text

.txt, .csv, …■ Software

ESRI■ Open

GeoTiffUSGS DEMLAS

―Binary―35% - 80% file size reduction depending on attributes

selected.

LAS Format

LAS Format

LAS Format

LAS Format

LAS Format

1-arc-second (30 meters) 1/3-arc-second (10 meters) 1/9-arc-second (3 meters)

Specifications

■ Other

Deliverables