1. Uncertainty and Best Practices
Point Radius Method
Uncertainty and
Best Practices 1

2. Sources of uncertainty:
Scale
Uncertainty (ft)
Uncertainty (m)
1:1,200
3.3 ft
1.0 m
1:2,400
6.7 ft
2.0 m
1:4,800
13.3 ft 
4.1 m
1:10,000
27.8 ft
8.5 m
1:12,000
33.3 ft
10.2 m
1:24,000
40.0 ft 
12.2 m
1:25,000
41.8 ft
12.8 m
1:63,360
106 ft
32.2 m
 1:100,000
167 ft
50.9 m
1:250,000
417 ft
127 m
20° 30’ N 112° 36’ W
Coordinate Uncertainty
Map scale
The extent of the locality
GPS accuracy
Unknown datum
Imprecision in direction measurements
Imprecision in distance measurements (1km vs. 1.1 km) 2

3. Best Practices Resources
MaNIS/HerpNET/ORNIS Georeferencing Guidelines
Guide to Best Practices in Georeferencing
manuals (

4. Sources of Uncertainty
The extent of the locality
GPS accuracy
Unknown datum
Imprecision in distance measurements
Imprecision in coordinate measurements
Map scale
Imprecision in direction measurements 4

5. Precision vs. Accuracy Disambiguated
Precision is…
…the level of detail contained in or described by the data.
Example:
42, precise
42.1, more precise
42.01, even more precise
Precision can help to minimize uncertainty.

6. Precision vs. Accuracy Disambiguated
Accuracy is…
…a measure of how close a given value is to the true value.*
Example: Truth = 42
= more precise, less accurate
*We may never actually know the true value in georeferencing, but we do our best to reproduce the location of the true location.

7. Precision vs. Accuracy Disambiguated
Final Answer?
The sum of what we know…
about what we don’t know…
is the uncertainty.

8. Precision vs. Accuracy Disambiguated
More Precise
Less Precise
More Accurate
Less Accurate x

9. The Extent of the Locality

10. GPS Accuracy 5m 9m 10

11. Error assuming NAD27 vs. NAD83 or WGS84
Unknown Datum
Error assuming NAD27 vs. NAD83 or WGS84 11

12. Imprecision in Distance Measurements12

13. Imprecision in Coordinate Measurements
Uncertainty based on coordinate precision using the WGS84 reference ellipsoid
20° 30’ N 112° 36’ W vs. 20° 30’ N 112° 30’ W 13

14. Imprecision in Coordinate Measurements
Uncertainty based on coordinate precision using the WGS84 reference ellipsoid
52° 30’ N 112° 15’ W? 14

15. Map Scale 1/30th (.033) inch 1/50th (.02) inch
USGS Map Accuracy
USGS Standard: "For maps on publication scales larger than 1:20,000, not more than 10 percent of the points tested shall be in error by more than 1/30 inch, measured on the publication scale; for maps on publication scales of 1:20,000 or smaller, 1/50 inch." 15

16. Imprecision of Direction Measurements
Error: N > NE > ENE 16