Lab 5: Introduction to GPS

Introduction

In this lab, students will learn the basics of using a Trimble Juno GPS and ArcPad to collect data in the field. A geodatabase and three feature classes will need to be made and loaded into the GPS prior to collecting data. The study area is UWEC's newly developed Campus Mall. Four designated polygons, a footbridge, three trees, and three light poles are too be mapped. This lab is only meant as an intro to GPS data collection for vector map creation.

A GPS unit collects position and elevation data at locations on the Earth's surface by utilizing satellites and ground monitoring stations. The monitoring stations are responsible for tracking and modifying satellite fight paths and monitoring and analyzing satellite signals. There are currently 27 active satellites in 6 different orbital planes.

Figure 1: Illustration of how satellites determine position and elevation.


For an accurate measurement of position and elevation at least 4 satellites need to be available for the GPS to connect to. These satellites transmit radio signals in two frequencies, one is public access and the other is restricted to military use. The transmission of radio signals can be visualized as spheres around the satellite. The position at which all 4 (or more) spheres met is the position of the GPS on the Earth's surface.

Figure 2: Illustration of the effects of satellite geometry.


The position of the satellites available also play a role in accuracy. PDOP stands for positional dilution of precision and is a measure of the effect of satellite geometry, or how the satellites are arranged in space. Low PDOPs indicate a wide spread of satellites which is ideal. High PDOPs indicate a clustering of satellites which can result in larger error.


Methods

First a geodatabase needed to be made. This was done by right clicking the desired folder, hovering over New and selecting File Geodatabase. Then in the catalog window, three feature classes were made by right clicking the newly created geodatabase, hovering over New and selecting Feature Class.... One polygon, one line, and one point feature class were created and given a text field called Type. Each feature class was given the statewide projected coordinate system NAD 1983 HARN Wisconsin TM (meters). These blank feature classes were then added to a black ArcMap document. An image of the Campus area was then imported into the geodatabase by right clicking the geodatabase, hovering over New and selecting Raster Datasets.... Finally, a campus buildings feature class was imported into the geodatabase by the same process above but instead of selecting Raster Datasets..., Feature Class (single) was chosen.

Figure 3: A screenshot of the map document ready to be sent to ArcPad. The color of polygons will transfer into ArcPad, so a bright yellow was chosen to stand out more on a small screen while outdoors.


To enable ArcPad Data Manager, its extension needed to be added by navigating to Customize > Extensions and checking the box next to ArcPad Data Manager. Next the ArcPad Data Manager Toolbar was added by navigating to Customize > Toolbars and selecting ArcPad Data Manager.

Figure 4: The ArcPad Data Manager toolbar. The first button that looks like a GPS unit with an arrow pointing towards the right, the Get Data For ArcPad button, is selected first. Once the data has been collected, the button that looks like a GPS unit with an arrow pointing towards the left, the Get Data From ArcPad button is selected.

Figure 5: Get Data For ArcPad was selected and Next was clicked on the welcome screen. Several things were changed in this window. Action was selected and hovering over defaults, the Background layer format was changed to AXF file and the Background layer editing was changed to Editing allowed. This will allow the feature classes made in the geodatabase to be exported as feature layers in a ArcPad AXF file. Then Action was selected again and Checkout all Geodatabase layers and copyout all other layers was chosen. Next was clicked.


In the next window the folder name, map name, and storage location were selected. Next was clicked. Making sure Create the ArcPad data on this computer now option was checked, Finish was clicked to end the process. A folder will be created where it was specified. This folder contains the background image, an AXF file which contains the editable layers, and a ArcPad Map (APM) file. The folder was copied and pasted in the same location for backup and then pasted again into the Trimble GPS unit. Everything was now ready for data collection in the field.

Figure 6: ArcPad Toolbars. The GPS was turned on and ArCPad 10.2 was opened. The screen looked similar to the one above. In the Main Toolbar (folder icon) the open map icon was chosen and the proper APM file was chosen. The icon to the right of the Main Toolbar icon opens the Browse Toolbar (hand above globe icon). Zooming and panning options are available here.

Figure 7: The Edit Toolbar (pencil with polyline icon). Here the pencil was chosen to begin editing, the desired feature layer was chosen, and the feature type icon on the toolbar (the third icon, looks like a point in this image) was changed to match whichever feature layer was chosen (point, polyline, polygon, etc). For points, once the feature type was selected, the desired features could be collected by selecting the add GPS vertex icon (next to the feature type icon). Once a set number of positions was taken, the GPS averaged the values and generated a single feature. The type field was then modified. For lines and polygons, once the feature type was selected, the Command Bar appeared at the bottom of the screen.

Figure 8: The Command Bar. To collect the desired line or polygon features, the Add a GPS Vertex Continuously (the rightmost icon) was selected. When the feature was complete, the Proceed to Attribute icon was chosen (second icon, big green arrow) and the Type field was modified.

Figure 9: For this Lab, the areas in red were the designated places to collect polygon data, the footbridge seen in the picture was deemed a line, and three trees and three light poles were chosen at random for points.

Figure 10: After the data was collected, the GPS was connected to the computer and the Get Data From ArcPad icon was chosen on the ArcPad Data Manager Toolbar. The green plus sign in the upper right corner of the Get Data From ArcPad window was selected and the appropriate APX file was chosen. The box next to each feature class was checked and the Check in button was clicked. The data collected in the field was now in ArcMap as vector data and a map was made.


Results

Figure 11: The resulting map from the GPS data collection activity. The polygons created were not spectacularly accurate. This could be remedied by slowing the pace of movement around the features to allow more points to be created. The inaccuracy of the data collected is also a result of the capabilities of the GPS used.


Sources

Blog
Figure 1-2 taken directly from class lecture slides
Figure 4-6 taken from the ArcGIS Resource Center. http://help.arcgis.com/en/arcpad/10.0/help/index.html#/Overview_of_ArcPad_toolbars/00s1000000wn000000/
Figure 7 taken directly from Lab 5 instructions.

Data
Aerial Imagery provided by United States Department of Agriculture National Agriculture imagery program 2013.
Campus buildings feature class provided by UWEC Department of Geography and Anthropology.
GPS data collect by Lee Fox.