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NOAA Datums Videos 2014

Differential correction with NA2011 and the newest epoch of NAD83

See our and Datums Basics and GPS Tutorials for the long-awaited results of months of rigorous testing and peer review

Trimble Positions software has replaced Trimble GPS Analyst in ArcGIS 10.1

ALASKA GPS and GIS users - check out Joel Cusick's Datum and Projections presentation Feb 2013

ArcGIS 10.1 - New and Cool improvements see Presentation from Melita Kennedy, ESRI : Projections and Datums Dec 2012

Home GPS Tutorials Key issues when moving GPS data to a GIS
Key issues when moving GPS data to a GIS PDF Print E-mail


Moving GPS data to a GIS

Users must pass through many datum gates to successfully collect and process GPS data.  The increasing availability of sub-meter accurate GPS, imagery and other spatial datasets is keeping users even more honest every step as once ‘minor datum shifts’ can no longer be overlooked.  Although a challenging topic for users of all levels, ignoring datums will introduce significant errors in the data you’ve worked so hard to collect.   

At each step, ask yourself:

1) to which datum is the GPS data currently referenced,

2) how might this step change that, and

3) to which datum is the resulting data file referenced.  

 Steps (and challenges) you will encounter when moving GPS data to a GIS

1. GPS data collection

Trimble TerraSync users are lucky as the data is also STORE DATA in WGS84.

ESRI ArcPad users BEWARE! Data can be shifted to another reference frame during data collection with ArcPad!

2. Differential correction

Key is realizing that your GPS data takes on the reference frame of the correction source

If you used a USGS beacon for real-time correction your data is now shifted to NAD83(2011). 

If you post-process your data with Pathfinder Office against a CORS base station, your data could be in ITRF00 epoch 1997.00

3. Export to GIS in one of millions of possible projections

Trimble's Pathfinder Office Differential Correction lets you configure an export setup. 

During this step, you choose a transformation to once again (possibly move your data to another reference frame. 

4.  Define your data's projection and use metadata

If you are moving your data to ESRI ArcGIS, a .prj file is required to define the reference frame which your data has acquired.

Trimble Pathfinder Office does an excellent job at generating .txt files to document every differential correction and export step. 

The ArcGIS metadata toolbar allows you to attach theese files to your data to keep track of chosen workflows.

Conclusion

Yes, mixing datums may inadvertently shift the data you have worked to collect with the utmost accuracy


DO NOT FRET!

Consult our Datum Gates series for step-by-step guidance for the following data collection and post-processing workflows.

Case 7 Trimble TerraSync , Pathfinder Office, uncorrected or WAAS, GPS

Case 8 Trimble TerraSync, Pathfinder Office, NDGPS

Case 3 ESRI ArcPad, GPS Correct and GPS Analyst, Uncorrected or WAAS

Case 4 ESRI ArcPad, GPS Correct and GPS Analyst, dGPS or OmniStar VBS