Some GNSS Musings


First things first – GNSS is the new GPS.  Actually, GPS is a subset of GNSS. GPS stands for Global Positioning System, GNSS stands for Global Navigational Satellite System. For decades folks refereed to any and all satellite navigation systems as GPS, and for good reason – the US Global Positioning Satellite system was the only game in town. However, the term ‘GPS’ properly describes just the global positioning system established and maintained by the United States. Now that the Russian GLONASS system is operational, and systems from the European Union, China and perhaps other players (India?) are coming on-line, the term for ALL space-based satellite navigation systems has shifted to GNSS.

OK, now that that’s out of the way.

I spent the last two days in training finally learning how to run Trimble’s TerraSync and Pathfinder Office software.  We’ve had TerraSync and Pathfinder Office software in our office for years, but never got any formal training on how to use either package.  The training was actually very good, and I can see now why a lot of surveying and engineering firms prefer TerraSync over GIS-centric packages like ESRI’s ArcPad.

The class was taught by one of the training and support personnel from our local vendor, NEI, and he did a great job.  Woven throughout the class are discussions about GPS, datums, coordinate systems and issues like unanticipated coordinate system shifts due to improper datum selection or datum mis-matches between the software and virtual reference station (VRS) datums.  We spent a good deal of time in the field actually experiencing the impact of changing datum selections in the software (for example, the shift seen when selecting NAD83 vs. NAD83 HARN).

So this class got me thinking again about GNSS and data quality and accuracy…

In the olden days, like before the turn of the century, these datum shifts generally didn’t concern GIS folks.  The shifts introduced by any datum mis-match were well within most folk’s error budgets.  In most cases we were ecstatic when GPS got us within a few dozen feet of the features we were collecting.  When the accuracy standard of the 1:50,000 topographic map you were using as a base was +/- 50 meters having GPS points a dozen or so feet off was no big deal.  In fact, we were tickled pink to be able to get that level of autonomous GPS accuracy.

Today things are much different. Improved GNSS software, antenna designs, the open availability of reliable GPS and GLONASS signals and the wide availability of GPS augmentation services like WAAS and local virtual reference stations (VRS) means that these systems are capable of sub-meter, often sub-foot, accuracies. That’s just for GIS data collection.  Survey-grade GNSS systems are capable of real-time accuracies to tenths of a foot. Suddenly datum shift errors of even one foot become very, very important for high precision data collection and surveying.

One of the biggest problems people in my line of work face is a general lack of understanding of GNSS in the GIS and civil engineering fields. In particular, many professionals lack up-to-date training and working knowledge of GNSS system capabilities, limitations and application to their line of work.  Evaluating and planning for the potential impact of things like datum shift on GNSS-based surveys or data collection projects is something they can’t comprehend largely because they haven’t been trained on it and, perhaps most important, have’t been forced to consider it when planning or managing a project.

Sadly, I’ve met far too many people with a GISP certificate hanging on their wall who couldn’t tell me the fundamental difference between the NAD 27 and NAD 83 datums, and I have yet to meet a single civil engineer who is not also a licensed surveyor who could explain to me the importance of knowing the datum his or her CAD drawing coordinate system is based on.  Yet both of these groups – the GIS professional and the civil engineer – have a fundamental interest in controlling the overall accuracy and precision of their work.  For the GIS professional it’s a matter reputation and trust.  For the licensed civil engineer it could be a matter of putting his or her work at legal risk.

If you work in the GIS field you can not call yourself a GIS professional unless you have a fundamental understanding of datums, coordinate systems and the importance of applying this knowledge to your workflows.  A strong knowledge of datums and coordinate systems is one of the foundational building blocks of our profession, and since so much of what we do these days is GNSS-based it makes it equally important to have a strong understanding of the impact different datum selections can have on the spatial quality of our data.

I’ve said before in this blog that those GIS ‘professionals’ who consider GIS to be little more than making web maps are headed to extinction. Here in the Atlanta metro area it would take me about an hour to hire a busload of web developers who can make web maps (and this includes time out for a stop at Starbucks). If that bus accidentally rolls into the Chattahoochee River and everybody drowns I can get another busload just as fast. However, the number of GIS professionals I’ve run into who can tell me the anticipated shift in State Plane (NAD83) and State Plane (NAD83 HARN) coordinates wouldn’t fill the first row of that bus.

For the civil engineering community the issue is less obvious but just as critical. GNSS-based surveying and data collection is becoming the norm on many projects. It is faster, cheaper and just as accurate as conventional surveys under the right conditions. This means civil engineers will be incorporating more and more GNSS-based data into their designs and relying on GNSS for jobsite control, machine control and as-built data verification. While the task of establishing project control, setting up survey equipment configurations and managing project survey requirements will fall to the the project surveyor, the project engineer still has overall responsibility for ensuring things are built to design.  If the project stakeout is a few feet out from the design drawings it may not be because the instrument operator has a hangover; it may be because the design work was done in one datum and the GNSS survey unit is set to another. Being able to identify a potential datum shift problem is a key skill for civil engineers working in today’s GNSS-based world.

– Brian

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