Using a GPS, Range Finder and Compass and Estimating their Uncertainty

Introduction

This lab introduces you to the primary tools that are used for collecting spatial data in natural resources research: GPSes, compasses and range finders. Each of these tools has it's advantages and disadvantages and you'll want to be able to select the right tools for a specific data collection task.

Note: if you are remote from HSU or decide not to participate in person, please find an open area with at least 8 features that can be seen from aerial images for this lab.

Learning Outcomes

By the end of this lab, you should be able to:

• Use a GPS Hand-held to gather data on the location of plants
• Bring data from a GPS and/or compass and range finder into ArcGIS and create maps
• Use a range finder and compass to find locations
• Describe the issues associated with accuracy of GPS units, compass, and range finder
• Estimate standard deviation and root-mean squared error (RSME) for coordinates
• Work in a team to collect field data.
  

Equipment

The GPS hand-held units we will be using are typical Garmin recreational-grade GPSes. The compasses are high quality field compasses and the range finders are relatively new laser units.

By far, the most critical step in using a GPS is to set the datum to WGS 84 and never change it! If you change the datum in the field, you will have data that is in two different datums and the GPS will not know which coordinates were collected with each datum. This makes the data unusable (I have seen this happen twice with field crews).

For this lab, you will need:

1. A GPS
2. A Range Finder
3. A Leveling Rod
4. A Compass
5. The Protocol and Data Sheet

Before Heading to the Field

We will be computing the uncertainty associated with field data collection so we need a point of reference, or benchmark, that we can see on a high resolution image. Your teacher will provide you with a high resolution image of HSU. Each group will use one of the four light poles along the north edge of the events field. These should be easy to see in the image and will be easy to find on the ground. Make sure you are using the center of the base of your group's light pole.

Field Data Collection with GPS

The instructions below are specifically for the Garmin 64st units. This video is for the Garmin eTrex units that remote students may be using.

• Turn on the GPS as soon as you are outside
• Set the datum to "WGS 84" and the projection to "UTM". For the Garmin 64st units:
  • Press "Menu" twice
  • Press "Enter" on "Setup"
  • Scroll down to "Position Setup" and press "Enter"
  • Use the scroll button and the "Enter" key to change the "Position Format" to "UTM UPS" and the "Map Datum" to "WGS 84".
• Note that changing the projection in a GPS does not change the way the data is collected or recorded as these are always in decimal degrees. This just changes what is displayed on the screen. However, changing the datum does change the data.
• There is no output for HDOP on these GPS units so we won't be recording it for this lab.
• To record a "point" just click the "Mark" button on the GPS. This will save the point as a "Way point" so they can be downloaded later.

Make sure you have the datum set to "WGS 84"!

Using the GPS, record the location of the other light posts around the events field. Also mark the location of at least 3 other features you are interested in. Make sure to return to your bench mark and record at least 3 way points so you can estimate the precision of your data.

Recording Data with a Compass and Range Finder

GPS handhelds are great when they work. However, they need a clear view of a large area of the sky to get a "fix". Here at HSU, and in many areas around the world, this is hard to come by either because you are in a canyon, have a lot of tree cover, need higher resolution than a GPS can provide or some combination of these factors. In addition, sometimes you may be able to see the features you want to digitize but can't get to them. In these cases, we need to find other solutions to get spatial data that meets our research needs.

YouTube video on using a compass and range finder.

Using the compass and range finder, "shoot" each of the other light poles from your benchmark. Also shoot the other 3 features you are interested in. Then, move to each of your light poles and shoot back to your original light pole. This means that for each coordinate, you will need to record the coordinate you were at (typically by name) and then the direction and distance to the new coordinate. Note that the protocol sheet has another set of columns for you to enter these values.

Note: You can either compensate for declination in the field or back in the lab. I recommend doing this in the lab as you'll have enough to worry about in the field.

Making a Map with GPS data

Back in the computer lab, follow the steps below. Note that if you are familiar with other ways of brining data into ArcMap from a GPS, you can use them. You just need to make sure you have the data correctly projected to WGS 84, UTM Zone 10 North when done.

The steps below use ArcMap, this video will walk you thought downloading the GPS data to ArcGIS Pro. Then, this video will show you how to prepare the data for analysis.

1. Create a folder structure in "D:\TempData" as we have done before:
• 01_Originals
• 02_Working
• 03_Final
2. Download the data from the GPS into the "Originals" folder. There are a variety of methods for doing this but I find the most reliable (for Garmin GPSes) to be:
   1. Connect your GPS to your computer using a USB cable.
   2. If you don't have DNRGPS installed on your computer:
      1. Search for DNRGPS on the web. This will take you to the Minnesota Department of Natural Resources DNRGPS (formerly known as DNRGarmin) web site.
      2. Download and unzip the software.
   3. In the DNRGPS folder, double-click on the "dnrgps.exe" program.
   4. From the "GPS" menu, select "Connect to Default GPS" and then "Find GPS". If you get an error, make sure the GPS is properly connected and has the appropriate drivers installed (this should happen automatically when the GPS is connect but it can take a few minutes).
   5. In the "File Menu" select "Set Projection" and make sure the projection is set to "UTM zone 10N". This will add two columns to your data that provide UTM Eastings and Northings.
   6. From the "Way point" menu, select "Download".
   7. You should see data appear in the "Way points" tab.
   8. From the "File" menu, select "Save to.." -> "File..."
   9. Set the file type to "comma-delimited" and save the data with a good file name somewhere you can find it.
3. In ArcMap, select "File -> Add Data -> Add X,Y Data"
4. Browse to the "CSV" file you created.
5. Make sure the "X Field" and "Y Field" columns that ArcMap selects are the "x_proj" and y_proj" columns, respectively.
6. For the "Coordinate System of Input Coordinates", select WGS 84, UTM Zone 10 North (i.e. Projected Coordinate Systems -> UTM -> WGS84 -> Northern Hemisphere -> WGS 84 UTM Zone 10 N).
7. When you click "OK", you may receive an error that the file does not have an "Object-ID" field. This is OK and the error will go away when we convert the data to a shapefile.
8. The data should appear in ArcMap. If you don't see the data, right click on the layer and select "Zoom to Layer". If you see any problems at this point, you may need to go back through the steps carefully. You can also check your CSV file in a program like "NotePad". Note that if you check the file in Excel, Excel may convert your values to "currency" and only show a couple of digits after the decimal.
9. These steps will create an "Event" layer in ArcMap 10.x. These layers are temporary and cause a bunch of problems. Immediately export the data to a shapefile by right-clicking on the layer and selecting "Data -> Export Data". Make sure to save the file in your "Working" folder, give it a good name that you'll be able to recognize in the future, and set the "Save as type" to Shapefile..
10. Now is a good time to select "File -> New" to clear out ArcMap and then load your new file. If you load it first, ArcMap should pick up the UTM zone and display your data in UTM. Move your cursor around the screen while looking at the lower right corner of ArcMap as the coordinates should be in meters. If your eastings are large negative numbers, you probably loaded the background first, try selecting "File -> New" again and load your UTM dataset first.
11. Add a map with an appropriate background and symbology.

Note: If you have problems with DNRGPS, try attaching the GPS and dragging and dropping the appropriate "GPX" file into BlueSpray.

Computing Coordinates From Directions and Distance

Add a higher resolution point using the 3 inch aerial data for Arcata as in this video.

Create an MS-Excel spreadsheet and add your field data and then additional columns until you have columns something like the following. The equations for new values in columns are provided.

• Benchmark Easting: From the aerial image (make sure these values in in UTM Zone 10 North)
• Benchmark Northing: From the aerial image
• Direction (make sure you have corrected for declination)
• Direction Radians: Direction / 180 * PI (convert degrees to radians)
• Distance: Horizontal distance in meters
• DX = sin(Direction Radians) * Distance
• DY = cos(Direction Radians) * Distance
• NewEasting = Benchmark Easting + DX
• NewNorthing = Benchmark Northing + DY

This video may help.

Analysis

To compute RMSE of your benchmark points you'll want to create a new spreadsheet. Add columns for:

• Easting: GPS easting value for all the benchmark points
• Northing: GPS northing value for all the benchmark points
• Benchmark Easting: From the aerial image (make sure these values in in UTM Zone 10 North)
• Benchmark Northing: From the aerial image
• EastingBias: Easting - Benchmark Easting
• NorthingBias: Northing - Benchmark Northing
• SqrEastingBias: EastingBias squared
• SqrNorthingBias: NorthingBias squared

Sum the SqrEastingBias and the SqrNorthingBias below their columns. These are Sum of Squares values. In cells below these values compute the RMSE values for the eastings and the northings by dividing the Sum of Squares values by the number of coordinates and then taking the square root.

You can use the MS-Excel function for standard deviation of a population to compute standard deviation.

This video may help.

Making a Map

To bring your coordinates into ArcGIS, you'll need to save them to Comma Separated Value (CSV) file. Also, ArcGIS is very picky about headings, blank rows, etc. I recommend making all headings 10 characters or less without any punctuation and just including the name of the points and the Easting and Northing. If you have problems importing the file into ArcGIS, examine it in NotePad to see if there is anything that might cause a problem. BlueSpray is much more flexible on importing files so you can also try dragging and dropping the file into BlueSpray and then saving the file to a shapefile that you can then bring into ArcGIS. Regardless, remember to define the spatial reference.

You may want to make two shapefiles from your data; one for the GPS coordinates and one for the compass/range finder coordinates. In ArcGIS, create a map that clearly labels the coordinates that were measured with the GPS and those that were measured with the compass/range finder.

Additional Resources

How to use a compass

How to use map and compass (video)

Creating Reports in MS-Word