Post-Processing

Discussion

After the field data collection, you will need to transfer data from the GPS to a PC and perform post-processing. All collected GPS data needs some form of post-processing whether it is differential correction and/or editing. After transfering, view your data on the PC for verification. While your data file is onscreen, you can identify any unwanted positions and/or features as well as attribute information and perform preliminary editing. Depending on your GPS unit and data collection software, this step may or may not include converting the data to a GIS format at the time of downloaded. Either way, it is important to have clear procedures for post-field data management and verification.

Steps to take when returning to the office.

  1. Transfer GPS files which may include GPS Data Files, base_files and almanacs into your project's file structure. See suggested project and data file structures in the Project Management & Data Dictionary Design Step.
  2. Run differential correction, if your GPS unit and PC setup has the capability. READ MORE >>
    Data collected with real-time differentially correction should not skip this step if their GPS setup has the capability. Depending on your equipment and real-time acquisition of either satellite or broad-band signals, you still may require post-processing differential to correct positions not corrected in real-time.
  3. Combine data files. At the end of a long GPS field day(s) you may have generated multiple files. Combining files allows you to edit multiple files together.
  4. Verify data onscreen. Display or verification of the data ensures the data "looks right" and is not a dataset left on a datalogger from someone else. Data display, depending on the software used, allows for viewing of the data in relationship with other background files, and has query functions to identify feature attributes.
  5. Perform preliminary editing. READ MORE >>
    Be sure to work on a copy of the raw data!!!  Depending on your GPS viewing software's capabilities there may be tools available to perform preliminary editing and assess quality. Editing tools are used to remove unwanted mistakes during collection, positions collected during un-cooperative satellite times and intersecting vertices between area or line features. Mistakes or errors not removed in this step, can be edited during the Finalizing GIS Data step after the GPS data is exported to a GIS format. GIS software packages usually provide more editing capability and flexibility.
    • Points require almost no editing, but may require close inspection. Does the point line up with other features in the GIS?  If misplaced, consult the notes for the feature and assess in-field quality issues that may have arisen (ie. poor PDOP, EPE, or operator error). Many times you may forget to close a point feature before walking away, thus creating a point hundreds of feet away from the intended spot.
    • Lines and Polygons may require extensive editing. Due to errors in GPS signals, or obstructions like canopy or large buildings, there may be cases where the road feature appears like a zig zag or the polygon feature collapses upon itself. Delete positions that deviate from the trend of a straight line, remove positions that cross over each other at intersections or delete a series of positions that were collected if you stood too long at a beginning of a trial or failed to pause during collection. (See a graphic of this!!). Graphic credit:  GPS Field School Training Manual - Cultural Resources GIS, May 1998
    • Attribute validation is also an editing function that can be used after the field to enforce attribute structure and ensure attribute values are filled in appropriately. Trimble users can edit attributes with Pathfinder Office Software if data was collected using a predefined data dictionary file (*.ddf). Autonomous Garmin users must utilize GIS software to enforce attribute structure after the data has been exported to a GIS dataset format.
  6. Clear out the GPS unit for the next job or person. Do not leave data files on the GPS unit if they have been properly downloaded and verified.
  7. Lastly, charge or refresh your batteries. Use this time when returning from the field to charge any internal or external batteries for your next trip. Refresh the supply of external batteries if charging is not an option.

Watch outs!

File management - A GPS field project may generate tens or hundreds of files over a course of a field season. Take time to set up a coherent file management scheme that organizes data transferred from the GPS to the PC and a logical sequence of file naming conventions that enable you to track file changes over time. See suggested project and data file structures in the Project Management & Data Dictionary Design Step.

File Transfer Troubleshooting - One of the most common problems with transferring data is the transfer flow rate through the computer's RS-2332 Serial port (COM 1 or COM 2). If a program like ActiveSync is holding onto the COM port, your GPS unit will not be able to transfer. Steps to free up a COM port include:

Do Not Delay! - In many instances you have plenty to do when you return from the GPS field trip, however it is imperative that you don't delay in editing and verifying the GPS data. This is best done before another day of data collection or after a project when the map in your head is still fresh. The longer data sits unedited, the less accurate the editing, the more suspect the information (GPS Field School Training Manual - Cultural Resources GIS, May 1998).

Combining Files - In order to combine multiple files together the data's file structure must be identical. For Trimble users this translates into using the exact same data dictionary file for every GPS unit and every data file collected. (Be sure to test the data dictionary before creating files to ensure compatibility and smooth integration in the GIS.)  For Garmin users, the same rule applies to shapefiles created in Garmin and exported to ArcView or ArcPad.

Reference Features - By collecting point features like anchors or beginning and end points, you can better edit zigzagging line features. Since points are always more accurate than an instantaneous position along a line (assuming you averaged that point), you can straighten out a line or close a polygon more accurately using the reference point feature as a guide or snapping feature in the GIS.

Glossary of Terms

Almanac - Data transmitted by a GPS satellite, which include orbit information on all the satellites, clock correction, and atmospheric delay parameters. The almanac is used to facilitate rapid SV acquisition. The orbit information is a subset of the ephemeris data with reduced precision.

Base files - GPS files collected continuous from either community base stations, local base stations, or Continually operating Stations (CORS). Gather base files will require an internet connection and a software that dials into a server that houses the base station data collected at the same time of the rover. Data stored on these servers will not be available in real-time - hence this step is post or after field collection. Trimble users would use the Differential Correction utility supplied in Pathfinder Office.

Differential Correction - The merging of rover file data with base map data to correct position errors due to atmospheric interference. You collected autonomous data (rover) in the field while base data is stored at the stationary base station. The two datasets are loaded into a post-processing software package where corrections are applied. This process will reduce errors in the field collected data (the rover), by correlating and correcting for known errors recorded in the base file that have the same time tag. As distance between the rover and base file increase there is a degradation in post-processed accuracy - in general the degradation of one part per million (1ppm) as the distance between the base station and rover increases. For example one millimeter of degradation occurs for every kilometer between base and rover.

Feature - A physical object or location of an event. A feature can be a point (a tree), a line (a trail) or an area (a wetland).

GPS Data files - Files collected on the GPS datalogger and require processing. Trimble users may have waypoints and rover files with an ssf extension. Garmin users may have collected points (waypoints), tracks (a series of trackpoints that can be converted to lines or polygons) or routes (a series of waypoints strung together). ArcPad users may be directly exporting shapefiles in either point, line or polygon feature format.

Links (Step-by-Step How To)

Base Stations: General Information: Recreation\Consumer Grade GPS Receiver Info: Trimble Information:

Site Suggestions. Contact ifer_mccollom@nps.gov if you have any suggestions or possible Step-by-Step How To links.