GPS Series: How to Collect the Best GPS Data—Part 4 ››
By: Jon Aschenbach - Resource Supply, LLC
“How To Obtain Sub-50 centimeter Accuracy”
A Five Part Series:
1. Back to Basics - Better GPS Accuracy For All GPS Units
2. Equipment Options for All GPS Budgets
3. How to Obtain Sub-meter Accuracy
4. How to Obtain Sub-50 Centimeter Accuracy
5. Are You Ready For Centimeter Accuracy?
This is the fourth part of a five part series on “How To Collect The Best GPS Data”. In the first
three parts we discussed GPS basics and a wide range of equipment options. This article narrows
in on achieving sub-50 centimeter accuracy. We will look at four basic methods:
1. Use a Sub-Meter rated GPS receiver with a real time correction.
2. Use a Sub-Meter rated GPS receiver with Post Processing.
3. Use a Dual Frequency GPS unit with the Omnistar or Atlas Subscription
4. Use a Dual Frequency GPS unit with the Canadian Post Processing system.
To consistently collect sub 50 centimeter data you are generally looking at collecting in an open
sky condition. Additionally, you will need to have a real time GPS correction coming in as you are
collecting your data, or you will need to do post processing.
Statistically, when I refer to sub-50 centimeter accuracy, I want that to be 2RMS (Root Mean
Square) which equates to a 95% probability. Many GPS systems get sub-50 centimeter accuracy
part of the time. In this article we are looking at a much higher consistency. Also, I am referring to
systems that I have personal familiarity with. There are probably many other brands and models
available.
Resource Supply, LLC
11607 SW Winter Lake Drive
Tigard, OR 97223
503-521-0888
Jon’s cell: 503-707-6236 Tyler’s cell: 503-547-3756
(Copyright 2014 by Resource Supply, LLC)
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Statistically, when I refer to sub-50 centimeter accuracy, I want that to be 2RMS (Root Mean
Square) which equates to a 95% probability. Many GPS systems get sub-50 centimeter accuracy
part of the time. In this article we are looking at a much higher consistency. Also, I am referring to
systems that I have personal familiarity with. There are probably many other brands and models
available.
Achieving 50 centimeter accuracy on a consistent basis is much easier in open sky conditions.
Still, it takes high quality equipment, good GPS protocol, and generally some means of applying
a correction to the original data. This correction can be a real time correction from a real time
GPS network such as the Oregon GPS Network or the Washington State Reference Network.
Corrections can also be obtained from the UNAVCO network of base stations which are part of
the Plate Boundary Observatory. The UNAVCO base stations are generally concentrated in the
Western part of the U.S. They will be discussed in an upcoming newsletter article.
Option 1: Using Real Time Correction Data Via Wi-Fi or Cell Phone. When we are lucky
enough to have cell phone coverage in the area we are working, the application of a real time
correction code can be pretty easy to set up. The GPS receiver must have the capability to be
used as a phone to receive incoming data, or it needs to have Bluetooth or Wi-Fi connectivity to a
phone. For this article, I am going to avoid listing the specific protocols and instructions needed
to set up the connection. I will just say that the real time corrections will be applied to the incoming GPS data through some sort of data transfer over the internet. The GPS receiver must be
capable of utilizing NTRIP and RTK. Both of these options may cost a few hundred dollars to activate.
Option 2: Post Processing. If you don’t have cell phone coverage where you work, don’t
despair! Post Processing can easily be done after you collect your GPS data. Post Processing is
handled somewhat differently between various manufacturers. Generally it is not difficult to do,
nor is it expensive. You do have to collect a specific data set that is used in the Post Processing
operation. This data set is raw GPS data that must be collected as you are collecting your main
GPS data. On Spectra Precision GPS receivers you can buy an option for Post Processing for
$495.00. It works in both ArcPad and MobileMapper Field software. The user turns on the recording of raw GPS data with a simple tap on the screen. .
In the Post Processing operation on a PC, a correction (distance, azimuth, and elevation) is applied to every second’s worth of data that you collect. Even if you have collected GPS data for
several hours, the Post Processing operation will be finished in a few seconds.
Shown below is a data set collected with a MobileMapper 100; errors are first listed without post
processing, and then after post processing. The four points being tested had OPUS solutions ran
on them and the known positions are accurate to about 1 centimeter. All of these points are located in areas that are quite open, although some 55 foot tall trees are across the street.
(Copyright 2014 by Resource Supply, LLC)
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GPS units rated at being sub-meter accurate in real time, will usually be sub-50 centimeter accurate with post processing. Accuracy can be as good as 10 to 20 centimeters after post processing with single frequency GPS receivers (ones rated at sub-meter without post processing).
Option 3: Use a satellite based correction system such as Omnistar or Atlas. Using a satellites based correction system such as Omnistar or Hemisphere’s Atlas system can work very
well when cell phone coverage is spotty or non-existent. With these systems, the user needs to
turn on their GPS unit and let it initialize with the satellite correction signal. This initialization process can take 8 to 30 minutes. Once initialization has occurred, the GPS receiver will maintain the
correction as long as the connection to the correction satellite is maintained. The GPS receiver
also needs to continually track GPS satellites. The main advantages of the satellite based correction services is that they can work anywhere in the world. Real Time accuracy of 4 to 12 centimeters is possible with a dual frequency GPS receiver . Different subscriptions have different accuracy levels. The subscription costs range in the neighborhood of $1,200.00 to $2,500.00 per year.
The main disadvantages of the satellite based correction systems are increased time for initialization and the subscription cost. If the user loses the correction signal under tree canopy, the receiver needs to go through the 8 to 30 minute initialization again.
Option 4: Use a dual frequency GPS receiver with the Canadian Post Processing System.
The Canadian Government offers service (at no charge) which allows any user to Post Process
data collected from a Dual Frequency GPS unit. While the minimum length of data collection on a
point can be as short as 3 minutes, I have seen very poor accuracy with files less than about 8
minutes. Ten minute files can provide 10 centimeter accuracy. Fifteen minute files can be closer
to 4 or 5 centimeters (sometimes near centimeter accuracy).
Can you afford to spend 10 minutes to collect a point? Maybe or maybe not. For property corners
and other important points, 10 minutes to get 10 centimeter accuracy might be a real bargain.
With my experience with the Canadian system, I would not even try it with less than 8 minute occupations. Also, remember that this service only works with dual frequency GPS units and relatively open sky conditions.
Continued on next page
(Copyright 2014 by Resource Supply, LLC)
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The major advantage that I see for using the Canadian system is that they accept files less than
15 minutes in duration (the minimum for OPUS), and they will give a solution when collecting near
trees. OPUS will reject files that appear to be “noisy”. The Canadian system works for the entire
US.
Here is the link to the Canadian Based GPS Correction System:
http://www.nrcan.gc.ca/earth-sciences/geomatics/geodetic-reference-systems/9052
So what does sub-50 centimeter accuracy cost?
Sub-meter GNSS Receiver with Post Processing
SXPad with SXBlue II GPS, ArcPad + Post Processing w GLONASS
$5,290
MobileMapper 120 Decimeter edition w MMF, GLONASS & Post Processing
$4,995
Sub-meter GNSS Receiver with Real Time Correction
MobileMapper 120 Decimeter edition (Has NTRIP, RTK, & GLONASS)
$4,995
(Costs do not include subscription fees for Real Time GPS)
Dual Frequency with OPUS or Canadian Post Processing
X90-OPUS receiver (Post Processing Only, No RTK)
$1,680
SXPad with Arrow 200 and SurvCE software & GLONASS
$8,690
ProMark 220 with RTK, NTRIP, GLONASS, and Fast Survey software
$8,950
Dual Frequency with Atlas Correction Service
Arrow 200, Nauitx X8, SurvCE (includes one year of 10 cm correction)
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$10,374
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Conclusion: Sub-50 centimeter accuracy is possible with good equipment, good GPS protocol,
and mostly open sky conditions. GPS equipment costs escalate rapidly with improved accuracy.
The next and final article in this five part series will focus on centimeter accuracy. We will look at
centimeter accuracy in real time and with post processing.
If you have any questions, please call me or Tyler Gakstatter at 503-521-0888.
Thanks!
Jon Aschenbach
Resource Supply, LLC
11607 SW Winter Lake Drive
Tigard, OR 97223
503-521-0888
Jon’s cell: 503-707-6236 Tyler’s cell: 503-547-3756
(Copyright 2014 by Resource Supply, LLC)
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