Novel Ground Truth Method for Crop Fields Combining a Global Positioning System (GPS) and a Handheld Geographic Information System (HGIS)

Geospatial Technology September 18, 2013 Print Friendly and PDF

Introduction

For the state of Arizona, Geographic Information System (GIS) map templates are created on an annual basis depicting the location of all potential crop fields.  These templates, along with a Global Positioning System (GPS) and Handheld Geographic Information System (HGIS), can be used to efficiently create a database of what crops were present during field visits and to assess the accuracy of crop classifications derived from remote sensing data.  These products can be easily created from inside a vehicle assuming that crop identification can be done without actually entering the field.  While employing this method, the GPS and map template will orient you to your location while in the field.  The HGIS software can be set to display the crop field map, allowing placement of GPS points directly in the crop fields of interest.  Once placed, the points can then be attributed accordingly.  Using two people, one to drive and one to navigate and “drop points,” many ground truth points can be collected in a short period of time.  This method cannot be used to digitize an area for the first time but can be used to assign a field type or other attribute(s) to a point placed in known and previously mapped fields or boundaries of any kind.

Protocol

Step 1: Gather and process background GIS data.

Begin by loading all necessary shapefiles into ArcMap or another desktop GIS platform.  A similar conversion can be performed using Google Earth and DNR Garmin, but conversion is simplest using the ArcGIS suite of tools.  The shapefiles may include, but may not be limited to, crop fields and roads files.  Confirm that all files are in the same projection.  Individual file size should not exceed 700 kb for the HGIS to efficiently handle the data.  If files are larger than 700 kb, it will be necessary to remove data from the shapefile or to “cut” the file into separate, more manageable areas.  This can be done in ArcMap by using the select element tool from the toolbar to highlight the elements of the shapefile to be cut. 

Once the correct elements are highlighted, right click on the shapefile which is being edited in the table of contents, select the data option, and then choose the export data option in the drop-down menu. 

Save all new shapefiles into a folder to be loaded onto a memory card. The new shapefile should be saved using the same coordinate system as the data frame.  Make certain that all files to be used jointly contain the same area. 

Step 2: Create attribute table for GPS data layer.

The next step consists of creating the attribute table that will be associated with the ground truth GPS points.  Using MSExcel or another spreadsheet program, create an attribute table containing headers describing data to be collected for each crop field.  These can include, but are not limited to, date, photo ID, crop type, crop height, and crop phenology.  Headers that exceed 13 characters will be truncated, but column width can be changed in MSAccess if necessary.  Columns should be formatted to text or number with a specific number of decimal places.  Drop-down menus can be created for each header by adding variables to the column cells beneath each header.  Variables, but not columns, can be added manually during data collection as well. 

Once complete, save the file as both .xls and .dbf to the same project folder as the exported shapefiles.  If changes are made, make them to the .xls file and then save over the old .dbf file.

Step 3: Prepare equipment.

The background shapefiles and GPS layer attribute table are now ready to be transferred to the memory card compatible with the handheld computer.  Saving all necessary files into one folder will facilitate the transfer of these data.  Remember that the shapefile is a combination of three or more files (.shp, .shx, .dbf, etc.) and that all must be copied for the shapefile to work.  Now you are ready to collect data.

Check that all equipment is functioning properly before heading out.  Bring along extra GPS and handheld computer units, cords, backup memory cards with your data, and batteries.  It is also helpful to print paper maps on which to chart the intended pattern of data collection and to allow for a “big picture” view of the study area.  Only a small area is clearly visible at one time on the handheld computer.

Step 4: Prepare for data collection: load shapefiles.

Once in the field, attach the GPS unit to the handheld computer and access the HGIS program.  Choose to start using GPS.  The default icon is difficult to see once all shapefiles are added.  Go to GPS on the toolbar at the top, select options in the drop-down menu, and choose a different symbol (the X, for example).  To import the crop fields shapefile, go to the file icon on the toolbar and select the load option in the drop-down menu.  Load the shapefile from the memory card choosing an appropriate color (Figure 1).  If the program asks for projection information, enter the same information that was used when the shapefiles were initially saved.  Repeat the process to import all shapefiles specific to the location being mapped.  At this point, your current location should appear on the HGIS map.

Figure 1: One map layer has already been loaded (crop boundaries), and the user prepares to load an additional shapefile.

Step 5: Prepare for data collection: load attribute table.

The final step before data collection is to load the attribute table to be populated.  In the upper right-hand corner of the screen, there is a bar that will alternate between being labeled "off" or "select one," based on the last action performed.  Select this bar, and a drop-down menu will appear.  Choose the draw option, and this will bring up a submenu of icons below the main toolbar.  Select the icon for point.  A menu will appear stating no working layer (Figure 2).  Choose the option “new layer from template.”  Another menu will appear asking for a layer name for the new point shapefile to be entered.  Select the point icon and then click OK.  You will then be asked to find your template on the memory card.  Navigate to your project folder and select the .dbf template created in Excel.

Figure 2:  The user has decided to create points and has to select a new template before proceeding.

Step 6: Data collection in the field.

Once the template is loaded, the collection of ground truth data can begin.  The GPS will indicate your location on the map using the symbol selected earlier.  Using this map, visually confirm your location by comparing the shape of the actual field to the shape of the map’s crop field.  If correct, use the stylus to place a point in the crop field on the HGIS.  While the point is still highlighted, select the attribute icon on the lowest toolbar.  The attribute table menu will appear for that point and will allow information to be selected/added (Figure 3).  Continue to add ground truth data and populate the attribute table in this manner.  The attribute table can be edited for any point at any time by using the “select one” option to select (highlight) the point of interest.  Choose the attribute icon on the lowest toolbar, and then make changes to the menu.

Figure 3:  The user has selected one of the created points to edit the attribute table for that crop field.  In this case, the user has selected alfalfa as the crop type from the drop-down menu.

 

As you drive or otherwise move through the study area, the map will change accordingly, keeping your location on the screen.  The pan, zoom-in, and zoom-out options are located in the drop-down menu allowing you to move the map as needed (Figure 4).  To save your data, select the file icon and choose save layer.  Be certain to select the correct shapefile and to save to the memory card, not the handheld computer.  Save frequently and always be aware of the battery life in the handheld computer.  Loss of battery power will result in the loss of any unsaved data and the loss of data saved to the handheld computer but not the memory card.

Figure 4:  All the tools the user can select including zooming in and out or panning around the study area.

This publication was made possible by contributions of the authors through projects funded by the Western Sustainable Agriculture Research and Education Program (Western SARE Subaward No. 080018004) and USDA-CSREES-NRI (Award No. ARZR-2007-02227).

Authors

  • Kyle Hartfield, Office of Arid Lands Studies, The University of Arizona, 1955 E Sixth St., Tucson, AZ 85719
  • Christa Ellers-Kirk, Department of Entomology, The University of Arizona, 410 Forbes Bldg., Tucson, AZ 85721
  • Kristen Wisneski, Office of Arid Lands Studies, The University of Arizona, 1955 E Sixth St., Tucson, AZ 85719
  • Barron J. Orr, Office of Arid Lands Studies, The University of Arizona, 1955 E Sixth St., Tucson, AZ 85719
  • Stuart E. Marsh, Office of Arid Lands Studies, The University of Arizona, 1955 E Sixth St., Tucson, AZ 85719
  • Yves Carrière, Department of Entomology, The University of Arizona, 410 Forbes Bldg., Tucson, AZ 85721

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.