The document discusses accuracy and precision of GPS/GNSS field data collection. It defines accuracy as how close data is to real-world values, while precision refers to how exact the data description is. Common sources of inaccuracy are satellite positioning, antenna placement, and body obstruction. Understanding the realization of the NAD83 datum used is important, as is knowing that NAD83 and WGS84 now differ by up to 2 meters in some areas due to updated definitions. A spreadsheet is recommended for checking accuracy of field data.

1. Bettering Our GNSS Data Collection:
How Do You Really Know the
Accuracy and Precision of Your
GPS/GNSS Field Data?

2. Mel Philbrook – Applied Geospatial Engineer Manager
• 2015 Worldwide Distributor of the Year
• Trimble Gold Channel Partner
• Over 25 Years Industry Experience

3. Agenda
• Accuracy versus Precision
• Common sources of inaccuracy
• Know your NADS
• What is a Datum Shift?
• Question and Answer Period

4. Accuracy vs. Precision
• Accuracy can be defined as the degree or closeness to
which the information on a map matches the values in
the real world. Therefore, when we refer to accuracy, we
are talking about quality of data and about number of
errors contained in a certain dataset. In GIS data,
accuracy can be referred to a geographic position, but it
can be referred also to attribute, or conceptual accuracy.
• Precision refers how exact is the description of data.
Precise data may be inaccurate, because it may be
exactly described but inaccurately gathered. (Maybe the
GIS Professional made a mistake, or the data was
recorded wrongly into the database).

5. Accuracy vs. Precision
Image A =
Is accurate and precise

6. Image B =
Is precise but not accurate
Accuracy vs. Precision

7. Image C =
Is accurate but imprecise
Accuracy vs. Precision

8. Image D =
Is neither precise or accurate
Accuracy vs. Precision

9. Common sources of added inaccuracy
• Where are the satellites?
• Know where is your devices antenna?
• Position your body for least obstruction

10. Where are the satellites?

11. You affect where are my satellites

12. Where is my antenna?

13. Understanding your NADS
• NAD83 has many realizations
• NAD83 (1986) Original
• NAD83 (1992) HARN
• NAD83 (1994) CORS
• NAD83 (2007)
• NAD83 (2011) Epoch 2010.00

14. What is a datum shift?
In the past we did not have to be concerned with the shift between NAD83
(1986) and WGS84 as introduced in 1987, because the discrepancy easily fell
within our overall error budget. NAD83 and WGS84, originally differed by only a
centimeter or two. That is no longer true. In their new definitions—NAD83
(2011) and WGS84 (G1762)—differ up to one or two meters within the
continental United States

15. Don’t believe everything you see

16. Truth Checker Spreadsheet
In the past we did not have to be concerned with the shift between NAD83
(1986) and WGS84 as introduced in 1987, because the discrepancy easily fell
within our overall error budget. NAD83 and WGS84, originally differed by only a
centimeter or two. That is no longer true. In their new definitions—NAD83
(2011) and WGS84 (G1762)—differ up to one or two meters within the
continental United States

17. Additional Resources
Frontier Precision Blog on High Accuracy GIS
https://fpimgis.wordpress.com/webinar‐high‐accuracy‐
gis/
Nightmare on GIS Street: Accuracy, Datums and Geospatial
Data
http://geospatial‐solutions.com/nightmare‐on‐gis‐street‐
accuracy‐datums‐and‐geospatial‐data/
GPS, GNSS for Geospatial Professionals (Penn State)
https://www.e‐education.psu.edu/geog862/node/1804

19. THANK YOU!
Q&A