Transcript Testing the Vertical Positional Accuracy of Digital

Testing the Vertical
Positional Accuracy of
Digital Elevation Data
Indiana GIS Conference
February 18, 2009
Robert W. (Bob) Wilkinson
Indiana DNR, Division of Water
Why Test for Vertical
Accuracy
• Does the data meet contract
specifications?
• Does the data meet your needs?
What About Horizontal
Accuracy?
The Only Deliverable in Our 2008
Mapping Was a Digital Terrain Model.
Past mapping experience has shown
that photogrammetric mapping with
good vertical accuracy will have good
horizontal accuracy.
Horizontal Accuracy
Office Review Looking for Problems Along Roads
Which Standard to Use?
National Map Accuracy
Standards (NMAS)
National Standard for
Spatial Data Accuracy
(NSSDA)
National Map Accuracy
Standards
Last Revised in 1947
90% of Test Points Within ½
Contour Interval.
What About the Other 10%?
Does the Map Meet the
Standard? (Pass/fail)
National Map Accuracy
Standards
Used by DNR 1965 - 1998
National Map Accuracy
Standards
Used by DNR 1965 - 1998
National Standard for
Spatial Data Accuracy
Issued by FGDC in 1998
http://www.fgdc.gov/standards/projects/FGDC-standards-projects/accuracy/part3/chapter3
National Standard for
Spatial Data Accuracy
Objective
The National Standard for Spatial Data
Accuracy (NSSDA) implements a
statistical and testing methodology for
estimating the positional accuracy of
points on maps and in digital geospatial
data, with respect to georeferenced
ground positions of higher accuracy.
National Standard for
Spatial Data Accuracy
Accuracy Testing by an Independent
Source of Higher Accuracy Is the
Preferred Test for Positional Accuracy.
National Standard for
Spatial Data Accuracy
The independent source of higher
accuracy shall be of the highest
accuracy feasible and practicable to
evaluate the accuracy of the dataset.
National Standard for
Spatial Data Accuracy
The independent source of higher
accuracy shall be acquired
separately from data used in the
aerotriangulation solution or other
production procedures.
Independent Source of
Higher Accuracy
Another
Map
Bench Marks from NGS
Bench Marks from DNR
National Standard for
Spatial Data Accuracy
95% Confidence Level defined as
1.960 * RMSEz
Reporting:
° Tested ___ Feet at 95%
Confidence
° Compiled to meet ___ Feet at
95% Confidence
Check Point Locations
Check points may be distributed
more densely in the vicinity of
important features and more
sparsely in areas that are of little or
no interest.
Check Point Locations
For a dataset covering a rectangular area
that is believed to have uniform
positional accuracy, check points may be
distributed so that points are spaced at
intervals of at least 10 percent of the
diagonal distance across the dataset and
at least 20 percent of the points are
located in each quadrant of the dataset.
National Standard for
Spatial Data Accuracy
If fewer than twenty points can be
identified for testing, use an alternative
means to evaluate the accuracy of the
dataset. SDTS identifies these alternative
methods for determining positional
accuracy:
° Deductive Estimate
° Internal Evidence
° Comparison to Source
2008 Division of Water
Location of 12 Projects
Also Included Bridge Surveys
Total Station
Beehunter Ditch at Linton
Sewage Plant Bridge
Beehunter Ditch at Linton
Sewage Plant Bridge
Bridge Section Points
Edited Points for Map Check
Surface Spot From ArcToolbox
Surface Spot From ArcToolbox
Table Before Surface Spot
Table After Surface Spot
Add and Calculate a New Field
Check Statistics
Import Table Into Excel
Re-check Mean, Max, and Min
An average that
is not close to
zero may indicate
a systematic
error.
Square the Errors
Calculate RMSEz and NSSDA
95% Confidence Level
Contract called
for 1.2 foot
accuracy at 95%
Confidence.
Map is Good
Accuracy Checks With
Simple Equipment
Accuracy Checks With
Simple Equipment
Accuracy Checks With
Simple Equipment
Accuracy Checks With
Simple Equipment
Windy Hill Subdivision in
Greencastle
Windy Hill Subdivision in
Greencastle
Windy Hill Subdivision in
Greencastle
Windy Hill Subdivision in
Greencastle
Windy Hill Subdivision in
Greencastle
Windy Hill Subdivision in
Greencastle
Check Level Notes in Excel
and Join Tables
Use Surface Spot
Command and Excel as
Before
Edinburgh East Side Swale
Edinburgh East Side Swale
New mapping tested 1.14 foot vertical
Accuracy at 95% confidence level
using the National Standard for Spatial
Data Accuracy
2005 Digital Elevation Model Tested
2.31 foot vertical accuracy at 95%
confidence level using the National
Standard for Spatial Data Accuracy
Brush Creek Reservoir
Brush Creek Reservoir
Brush Creek Reservoir
Brush Creek Reservoir
2005 Digital Elevation Model Tested
8.45 foot vertical accuracy at 95%
confidence level using the National
Standard for Spatial Data Accuracy
Metadata says it should have tested 6’
This was possibly the worst possible
location for testing the 2005 Digital
Elevation Model. Very thick brush and
near vertical cliffs.
Questions?