Abstract
Introduction
Background
Methods
Results
Analysis
Conclusion
Errata
References

Abstract

Introduction

Background

"Before mapping can begin, a network of accurately
measured geodetic control points must be defined
on the ground. Geodetic control points
...are the fixed starting points which surveyors,
photogrammetrists, and others use to determine two
or three dimensional positions..".Robinson et al., 1995

"...In general, these elevation points did not coincide
with the horizontal control network points being determined
by triangulation. Thus, we currently have two control networks-
horizontal and vertical."Robinson et al., 1995

Methods

Results

1. The eTrex Summit gave altitude readings that, in general, were lower than the control
   figures. And the 12XL gave readings that usually were higher than the controls.
   This was true regardless of the weather or the calibration status.
2. Without manual calibration of the eTrex Summit, the 12XL gave more accurate readings.
   
3. During stormy weather, the 12XL gave more accurate readings,
   even with manual calibration.

Analysis

1. COMPARING THE UNITS
   A number of factors make comparisons difficult between these two GPS units.
   
   • The number of points actually recovered in this project is relatively small.
     Furthermore, the terrain was relatively flat and close to sea level.
     Add to that some inconsistencies on the part of the user.
   • The user manual for the eTrex Summit states that its accuracy is plus or minus 10 feet...
     "Subject to proper user calibration." It further states that ..."For optimum
     performance of the Altimeter you should perform the calibration procedure
     entering either the correct elevation or barometric pressure." But the manual
     doesn't say how often or when to do this. A question to Garmin's e-mail
     Help Desk gave the following response:
     "It's a good idea to calibrate the altimeter the first time you use the Summit each day.
     Calibrating by a known altitude is best; otherwise, use barometric pressure at
     mean sea level for your area (usually your local airport can provide this)."
     The user manual for the 12XL doesn't state a vertical accuracy, and an e-mail
     query gave this answer:
     "The vertical accuracy of a 12XL varies between 50 to 150 feet."
2. FINDING CONTROL MONUMENTS
   Finding control monuments was very time consuming. USGS topo maps
   were very problematic. I couldn't find the monuments, or they seemed
   to be located in a slightly different location, or they were stuccoed over(?),
   etc. The National Geodetic Survey datasheets were much more helpful,
   but not without omissions, and several of the monuments listed by them
   were not recovered. The City of Sacramento vertical datum gave the most
   consistency in terms of there actually being a monument present,
   but suffered from using monuments or markers that did not bear a unique ID.
   Once a monument was found it may not have been possible to get a GPS
   reading. For example, construction scaffolding around the Post Office at
   8th and I St. prevented a clear sky view.
3. WHICH DATUM TO USE?
   The City of Sacramento ties its vertical datum to four monuments in
   downtown Sacramento. These are USGS monuments which use NGVD 29
   as their reference. On the other hand, the National Geodetic Survey uses NAVD 88
   where available, but does include NGVD 29 figures under the
   "Superceded Survey Control" section. Neither GPS unit permits selection
   of a vertical datum for its calibration or display.This study used the NGVD 29
   figures where available. Differences between the two datums is 2-3 feet

Conclusions

1. COMPARING THE TWO UNITS
   Generalizing about the relative vertical accuracy of the Garmin eTrex Summit
   and the Garmin 12XL isn't justified on the basis of this study. The points
   recovered lacked in number, variety, and conditions of use. However, the data
   does suggest that the Summit may be less accurate than the 12XL under
   certain circumstances, those being stormy weather and failure to manually
   calibrate the Summit on the day of use.
2. PERSONAL NOTE TO GIS 26 STUDENTS
   
   • My MAIN PROBLEM with this project was lack of a clear problem statement.
     I changed course in midstream several times, and did a lot of work for naught.
     
   • I should have done a field test of a small number of Benchmarks prior
     to searching for them. As it was, I first poured over topo maps,
     planned routes to travel, and researched a lot of points,
     and then went in the field, only to discover that a large
     percentage of the monuments weren't there.
   • The field work was fun but I should have spent more time planning in the office.
     I found myself in the field changing plans, revising my Field Notes sheet (which
     I never was happy with), and in general wasting time. Carrying around two GPS units,
     my clipboard, SPARE BATTERIES AND PENCILS, an umbrella,
     and assorted other gear was awkward. Itcould easily have been a two person job.
     And if you're collecting points along railroad tracks, watch it!

Errata

1. I want to thank the following for help with this project:
   
   • USGS personnel in Denver, Menlo Park, and CSU Sacramento.
     Especially helpful was Terry Carr, Cartographic Technician
     for the USGS working in the Menlo Park office.
   
   
   • NGS personnel, espcially Marti Ikehara, Geodetic Advisor with the NGS
     and working with the CA Dept. of Transpotation in Sacramento.
   
   
   • The staff for the City of Sacramento Dept. of Public Works
     and especially Roy Heavenston, Supervising Surveyor for the
     City of Sacramento.
   
   
   • The technical support team for the GARMIN Corporation,
     which promptly answered e-mail queries regarding their products.
2. Using USGS topo maps to locate geodetic monuments is very problematic.
   My initial searches for points involved exclusive use of these, and I went
   on many wild goose chases. Some were due to a lack of understanding on my part
   E.g., I didn't realize bench mark monuments could be placed in vertical aspect
   on the walls of buildings. But about half the time the monument just
   was'nt there. Why? It seems that monuments on walls are prone to being
   stuccoed over or removed when the walls themselves are torn down.
   I would often find an X marking a location where I believed a monument
   should be. A cartographic technician at the USGS office in Menlo Park
   told me that when streets are recurbed, the monument is often lost. He also
   told me that the USGS no longer maintains a Field Unit, so checking monument
   locations on any kind of regular basis just isn't feasible. I also found
   monuments through other sources that were present when the topo maps
   were originally made, but that weren't present on the ground. A good
   example would be the bench marks at either side entrance to
   the Post Office in Sacramento on I ST. between 8th and 9th streets
   (which are used by the City as the basis for its datum plane).


3. Searching for geodetic monuments can be an excellent source for local history.
   For example, the Citrus Heights topo map has a horizontal control mark named
   "Swanson". On the NGS datasheet for this mark, PID# JS106, it states that the
   monument is" ...on farm land owned by a Mr. A.C. Swanson ..." and that
   ".. .the route from here will depend on the crops that are being raised at the time...".
   While I was at that mark, an old timer walking his dog came up to me and
   started chatting. He not only told me something of the history of the Swansons,
   but informed me of the presence of another Benchmark on the opposite side
   of the street which did not appear on my topo map.
4. Walking along railroad tracks is an urban adventure. I recommend it to anyone with time
   on their hands and a paid-in-full life insurance policy.
5. Click HERE to see that searching for geodetic control
   points isn't all work and no play.(Can you spot the Benchmark?)
   

References