Technical Details: How to Use a Handheld GPS to Display State Grid Coordinates - Part Two
Professional Surveyor Magazine - June 2012
Read Part One Here
This series demonstrates the procedure for using a Garmin eTrex Venture Handheld GPS to display the Oregon Coordinate Reference System of 2011 (OCRS–2011, Transverse Mercator Projection) coordinates in either meters [m] or international feet [ift]. Part one of this series covers a more complex version of this procedure.
The process outlined here can be applied to any Transverse Mercator coordinate system given the appropriate parameters. While the Garmin eTrex will work with over 100 worldwide coordinate systems, it will not function with the Lambert Grid that many states use. However, you may create a local user-defined grid based on the Transverse Mercator projection to overlay the Lambert Grid area (caution: accuracy degrades as you move away from the created grid origin).
No warranty is given for the included procedures, and the user assumes full responsibility of verification of procedures and computations. The use of information contained here is at the user’s sole risk and without liability, risk, or legal exposure to the author, referenced sources, technical advisors, and/or Professional Surveyor Magazine.
If you’re having difficulty solving the equation from part one of this series, here is the promised short-cut method by Steve Weible, LS, for setting up the Garmin to display Eugene Zone metric [m] grid coordinates (using the computing power of the Garmin to accomplish this series of complex calculations), which yields practically the same results.
In the second section of this article, we’ll use a procedure explained in an article by Dale DePriest that yields international feet [ift] grid coordinates (see the OCRS-2011 section). The procedure avoids the task of multiplying the metric values by that pesky constant of 3.280 839 895 013.
When we use Steve’s method and then compare with the precise meridian distance computations (by Michael Dennis, RLS/PE, geodesist, referenced in part one of this series), the results are within 1 ift (the difference is due to the rounding of decimal values and the available Garmin grid coordinates display precision). This is well within the manufacturer’s stated accuracy and satisfies personal field-testing requirements.
Note: the Garmin displays X above Y or East Coordinate first and North Coordinate second.
First, we’ll need the parameters of the OCRS - 2011, Eugene Zone (NAD83, Transverse Mercator).
Latitude of Grid Origin: 43°45’00” N (43°45.000’ N)
Central Meridian: 123°10’00” W (123°10.000’ W)
False Northing: 0.000 m
False easting: 50 000.000 m
Central Meridian Scale: 1.000 015 (Exact)
We’ll set up the Garmin with System Setup functions (Figure 1).
Important: Before you can obtain grid coordinates directly on your Garmin, the difference in Northing between the UTM origin and the Eugene Zone origin will have to be determined. The UTM Zones within the northern hemisphere have their Grid Origin at the equator with a False Northing of 0.000 m. The resulting difference is entered as a negative value for the required False Northing value that will make the Y (Northing) coordinates display correctly in meters. Following the procedure described below, you should come up with a value of 4 846 118.0 meters. Therefore, the False Northing that you will have to enter into the Garmin is –4846118.0 m (notice the negative sign preceding the value), the distance from the Eugene Zone grid origin to the equator (Figure 2).
Setting Up the Garmin User Grid for the Eugene Zone Metric Coordinates
- With the Garmin eTrex Venture turned on, press the Page key at the upper right of the unit until you get to the Main Menu screen.
- Using the click-stick for movement and selections, scroll down and select Setup.
- Scroll down and to the right and select Units.
- On the Units Setup screen, review the available choices and select the following information:
- Position Format: hddd°mm.mmm’
- Map Datum: NAD83
- Distance/Speed: Metric
- Elevation (Vert. Speed): Meters (m/min)
- Depth: Meters
- Using the page button (upper right of unit), press twice to return to the Main Menu.
- Scroll to the right and select Mark (which will take you to Mark Waypoint).
- Scroll to the top box (which is the Waypoint identification [I.D.] box) and highlight. (If you desire, you may change the waypoint symbol that appears at the left side of the box.)
- Then, using the drop-down alphanumeric keypad, enter the desired point number and a short description for the Waypoint (we‘re naming the waypoint 101 Origin).
- Scroll down to the Note box and edit or enter information as desired.
- Scroll down to the Location box, select the box and enter. Enter latitude as N43°45.000’ and longitude as 123°10.000’ (Figure 3).
- Scroll down to the Elevation box and enter the elevation of the waypoint as 188 meters (in this example, information scaled from a USGS topographic map at lat/long location).
- Scroll down to the lower right, highlight OK, and press the click-stick. This should return you to the Main Page. If not, use the page button (on upper right of unit) to return to the Main Menu.
- Scroll down to Setup and press the click-stick.
- Scroll to the right, select Units (this will take you to Units Setup page), and select Position Format.
- Scroll through choices and choose User UTM Grid (Figure 4).
- Scroll down to the box labeled Longitude Origin and select this box.
- Using the drop-down numeric keypad, enter the longitude of the Central Meridian for the Eugene Zone, which is W123°10.000’, and select OK.
- Scroll down to the box labeled Scale and select this box.
- Using the drop-down numeric keypad, enter the Scale factor at the Central Meridian of the Eugene Zone, which is +1.000015, and select OK.
- Scroll down to the box labeled False Easting (m) and select this box.
- Using the drop-down numeric keypad, enter the Easting Coordinate (X) value of the origin, which is 50000 m (entered as +50000.0).
- Using the click-stick, scroll down to the box labeled False Northing (m), enter the Northing Coordinate (X) value of the origin, which is 0 m (entered as +00000.0), and press the click-stick. Finally, scroll down to the Save button and press the click-stick.
- Now you may return to the Main Menu by pressing the page button.
- Scroll right to Find and select Find on the Main Menu.
- Select the Waypoints icon and then select the Waypoint (101 Grid Origin) you established for your grid origin by pressing the click-stick. The screen should display Location as 0050000 (X) and USR 4846118 (Y) (Figure 5).
- Press the page button three times to return to the Setup icon.
- Select Setup and then scroll right to Units.
In the Units Setup screen, select the Position Format box.
- Choose User UTM Grid.
From the drop-down list, scroll down to User UTM Grid and highlight it. User Grid should be displayed at the top of the screen.
Scroll down to the box labeled False Northing (Y) and select this box.
- Using the drop down alphanumeric keypad, enter the False Northing (Y) value (changing to a negative number) that was determined using the previously described procedure, –04846118.0, and select OK.
- Scroll down and highlight the Save button and select Save.
- Screen Display of the User Grid Page
- Longitude Origin: W123°10.000’
- Scale: +1.0000150
- False Easting: +50000.0m
- False Northing: –4846118.0m
- Using the page button, return to the Main Menu page.
Using the page button, scroll to Find, select Waypoints, and view Waypoint list.
- Scroll through the list and then select Waypoint 101 (Grid Origin of the Eugene Zone). The display should read X 0050000 and Y USR 0000000 (Figure 6).
- Press the page button three times to return to the Main Menu page.
- Scroll to the Satellite icon and press the click-stick to display the location page. This page lists satellites (with numbers) available at the bottom of the screen, signal strengths (vertical bars), WAAS-enablement (“D” within vertical bars), position in OCRS-2011 Eugene Zone in X and Y metric coordinates (or in lat/long, if selected) in upper-right corner of display, and approximate location position accuracy in upper-left corner of display (Figure 7).
Trick” the Garmin to Display Inter-national Feet Grid Coordinate Values
OCRS-2011 – Eugene Zone, Modified to Display International Feet
Following the procedure we used to obtain metric values, we’ll replace some of the parameters and use the computing abilities of the eTrex Venture HC to “trick” the Garmin, and voilà, we have grid coordinates in international feet (the real, everyday Oregon survey measurements!).
- Reset the screen display of setup and units for feet (Figure 8).
- Highlight Position Format and User Utm Grid.
- Press the click-stick to display User Grid.
- Using the drop down alphanumeric key pad, replace the Central Meridian Scale with: 1.000 015 [from Eugene Zone parameters] * 3.280 839 895 013 = Modified Scale Factor to be entered in Garmin as C.M. Scale Factor of 3.280 889 107 611 (rounded to +3.2808891 for Garmin entry).
- Then replace the False Easting (X) and the False Northing (Y) with: False Easting: 50 000 m * 3.280 839 895 013 = 164,041.994 ift (rounded to +164,042.0 for entry). False Northing: –4 846 118.0 m * 3.280 839 895 013 = –15,899,337.270 ift (rounded to –15,899,337.3 for entry).
- The User Grid should now display:
- Longitude Origin [W 123°10.000’]
- Scale [+3.2808891]
- False Easting [+164042.0m]
- (Although the Garmin eTrex Venture HC
displays as m, the value shown is really ift.)
- False Northing [–15899337.3m]
- (Although the Garmin eTrex Venture HC
displays as m, the value shown is really ift.)
- Scroll down and highlight the Save button, and select.
- Return to the Waypoints list and select Waypoint 101 (Grid Origin of the Eugene Zone). The resulting display values: X 0164042, Y USR 0000000, and elevation of 617 ft (Figure 10).
Practical Verification of Results
The results were compared with precise GPS latitude and longitude observations on Foster Dam (Sweet Home) observation pier USACE “FD-1” (PID: BBBH53), conducted by USACE on June 4, 2009. Lat/long was translated to the OCRS-2011 Eugene Zone projection (rounded to the nearest single m or ift unit). Meters: X=90 045 m and Y=73 416 m, or International Feet: X=295,425 ift and Y=240,865 ift (Figure 11).
Further verification was obtained by observing Oregon Supernet Station “LANG-1989” located in northwest Lebanon and comparing with May 1992 NAD 83/91 precise observation values. The readings were recorded when the satellite signal strength bars displayed a “D” (“D” indicates Wide Area Augmentation System [WAAS] enablement.) Setting the Garmin Units Setup to User UTM Grid units for my observations, the X (East Coordinate) and Y (North Coordinate) averages were generally within the Garmin Estimated Position Error accuracy of three meters (10 ift) 95% of the time.
Weible, Steven E., LS. "Getting More from Your Handheld GPS Unit," The American Surveyor, Vol. 5, No. 9, October 2008.
DePriest, Dale. “Datums, Grids, and Other Measures,” 2002. www.gpsinformation.org.
Oregon Coordinate Reference System: Handbook and User Guide, Version 2.00, March 25, 2011.
An Introductory Venture HC Handbook for Natural Resources Managers, Virginia Tech, 2009.
Hank Berg, PLS (retired), began his career with the 880th Engineer Battalion (Heavy Construction) MOANG in 1957 as an engineering drafter and finished his enlistment as a topographic and construction surveyor. Prior to retiring, he held registration as a land surveyor in seven states. You can reach him at Hank_Berg@hotmail.com.
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