GPS for GIS Data Collection – 101: Webinar Follow-up

March 3, 2010  - By

Thank you for making “GPS for GIS Data Collection – 101” one of the most well-attended webinars we’ve done. It’s the first that was co-hosted by GPS World magazine and Geospatial Solutions online. If you don’t subscribe to my Geospatial Solutions Weekly newsletter, you might want to consider it as I venture into GIS and broader issues that I don’t have the space to cover in this newsletter. Also, the webinar had a record number of sponsors. Thanks to Hemisphere GPS, Laser Technology, and First American. Those folks make it possible for us to bring these webinars to you free of charge.

As customary, the newsletter after the webinar is dedicated to addressing some of the questions and posting the results from the polls I took during the webinar.

Poll Results

I conducted three polls during the webinar. I received some feedback that we aren’t giving folks enough time to respond to the polls. We’ll pay more attention to that in future webinars and allow more time. Following are the results:

Poll #1: Do you currently use GPS for collecting GIS data?

Yes:     68.5%
No:     31.5%

Total votes: 165

Poll #2: What accuracy do you require in a GPS mapping system?

cm-level:     28.4%
One foot:     10.8%
Sub-meter:    33.1%
1-3 meters:    22.3%
3-5 meters:    4.1%
5+ meters:     1.4%

Total votes: 148

Poll #3: Select the three most important items to you in a GPS mapping system.

Collect attribute data:    88.1%
Cost:                71.4%
Ergonomics:            7.9%
Photo-geotagging:        19.8%
Accuracy:            87.3%
Laser offset points:        22.2%

Total votes: 126

Question #1: How many satellites are transmitting and how many are just for replacement purposes?

Gakstatter: There are 30 operational GPS satellites. Currently, they are configured in a 24-satellite configuration so six of them are orbiting as “back-ups.” There are also three satellites, I believe, that are in inactive reserve that could be brought back into service if required.

However, as covered in my last three newsletters, the DoD is transitioning the GPS constellation to a 27-satellite configuration to improve satellite visibility to users. The process of transitioning started in January will take up to two years to complete. Please see the following articles for details on the 24+3 configuration:

The New GPS 24+3 Constellation: What Does it Mean to the Surveying and GIS User?

GPS 24+3 Configuration: A Closer Look

The Best and Final Look at the GPS 24+3 Configuration


Question #2: I do have a question, but it will take too long right now. How do I contact you later?

Gakstatter: Please feel free to e-mail me with questions any time… I learn a lot from your questions.


Question #3: What about use of iPhones or Blackberries with GPS embedded in the device?

Gakstatter: As smartphones become more powerful and prevalent, I think the use of them for GIS data collection will increase. I have two comments on this:


  • To this point, the ability to run GIS data collection software is hit or miss. Some smartphones just don’t have the resources (memory, processing speed) to handle running the more powerful data-collection software on the market. Of course, with technology advancing that may not be as much of an issue in the future, and it’s possible that GIS software manufacturers will write streamlined software specifically for smartphones.
  • The accuracy of GPS receivers built into smartphones will always be pretty rough. I’d put it in the 5+ meter category and I don’t think it will get much better, so adjust your expectation accordingly. However, using Bluetooth you might be able to “tether” the smartphone to a higher performance external GPS receiver.


Question #4: Is there a place for consumer-grade receivers in GIS data collection?

Gakstatter: Yes, I wrote an article on this last year. You can read it here…

Consumer-Grade GPS Receivers for GIS Data Collection

Please don’t hesitate to e-mail me more questions about this that may not be answered in the referenced article. I’ve been thinking about a follow-up article on this subject.
Question #5: What accuracy would you expect to record from a GPS handheld unit?

Gakstatter: There are high-performance handheld GPS receivers that can deliver centimeter-level positions and there are consumer-type handheld GPS receivers that delivery 5+ meter accuracy. This is typically a direct relationship between accuracy and cost (you’re not going to get sub-meter accuracy from a $200 receiver).

The best way to approach this is to decide what accuracy you require (cm-level, one foot, sub-meter, 1-3 meters, 3-5 meters, 5+ meters) and look at the budget you have available. You might want to take a look at the webinar I conducted last year titled “A Buyer’s Guide to GPS/GIS Mapping Equipment” and a newsletter article I wrote around the same time titled GPS Receivers for GIS Data Collection.



Question #6: We have a Topcon GMS-2 unit using an exteral antenna on a range pole similiar to one of the pictures you had in the presentation. How does the height of the range pole with the external antenna affect the X-Y position? Or does it? Thanks.

Gakstatter: The value of the range pole is that it gives the GPS antenna a clear view of the sky (above your head and other local obstructions). It can only improve your X-Y position. I don’t know how many times I’ve seen users hold a handheld GPS receiver up against their chest, effectively eliminating the use (and degrading accuracy) of GPS satellites behind them.


Question #7: For area determination which is preferred: static or dynamic?

Gakstatter: Personally, I would use dynamic unless you’re talking about a very small parcel of land (less than an acre). I’ve seen a number of reports on this and I believe all of them used dynamic data collec
tion with pretty reasonable results. In other words, I don’t think static buys you much in terms of acreage precision. However, I’ve been in circumstances where I used a combination of both such as when I know there’s a reasonably straight line between two vertices, but it would be very difficult to walk a direct line between them. In that case, I might use static for that leg of the traverse.


Question #8: I thought that PDOP was Positional Dilution of Precision.

Gakstatter: Several of you busted me on this. I mis-typed the presentation slide. I wrote Precision Dilution of Precision, which doesn’t make any sense. It should have been Position Dilution of Precision (PDOP). The horizontal component of PDOP is HDOP (Horizontal Dilution of Precision). The vertical component of PDOP is VDOP (Vertical Dilution of Precision).

Click here for a Wikipedia link that provides a little more information on GPS DOPs.


Question #9: Explain limitations of what type of project you cannot do if not a licensed surveyor.

Gakstatter: Because local laws vary widely, it really depends on where you are working. Even within a country like the U.S., each state has its own statutes that define the roles of the land surveyor.

In some areas, activities as simple as GIS data collection must be supervised by a licensed surveyor. In other areas, high-liability activities such as construction staking can be done by virtually anyone.



Question #10: Could the steel plate in my head cause multipath or obstruct signals when I use the integrated antenna?

Gakstatter: I can safely say (tongue in cheek) that in 20 years of GPS product development, conducting workshops/seminars, attending conferences, and performing GPS fieldwork, I’ve never heard this question. I’m speechless.  🙂


Question #11: A presumption that we should avoid is that by default “GIS data collection” implies low accuracy. This is simply not true. Position accuracy is independent of GIS. GIS can handle any level of accuracy the user desires. There is no such thing as a “GIS-grade” or “GIS-accuracy” survey. What relationship does GIS have with accuracy?

Gakstatter: I think Guest Commentator Craig Greenwald and I covered this well in the webinar, but it’s good to reinforce the point. I cringe when I hear someone say GIS stands for Get It Surveyed because it implies that the quality of a GIS is dependent on accuracy. It’s not. In some cases, +/- 500 feet. accuracy is perfectly fine for analysis in a GIS. The accuracy required by a GIS totally depends on the type of analysis you are conducting. Many surveyors typically think of GIS in terms of a land record (parcel) mapping system, but GIS is used for so much more than that. You don’t need cm-level accuracy to find the optimal location for the next McDonald’s restaurant within a city.


Question #12: Do you plan on conducting a webinar that will discuss strictly GPS, i.e., RTK vs. static, data reduction, post processing, etc.

Gakstatter: Yes, if you’re not subscribed to the Survey Scene newsletter, please sign up for that here as well as the Geospatial Solutions Weekly newsletter on the same sign-up page. The price is right…free. You can also look at the webinar archives where I have covered some of these subjects before. I’m also scheduled to conduct at least three more webinars this year (next one in May/June – topic not yet determined).


There were many other questions and I’ll continue including answers to them in the mid-March Survey Scene newsletter. Also, I suggest you sign up for my Geospatial Solutions Weekly newsletter (GSS Weekly) as mentioned above as I tackle GPS/GIS-related issues there, too. Next week, in the GSS Weekly, I’ll continue my discussion on the roles of the surveyor and GIS professional.


Thanks, and see you next time.

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About the Author:

Eric Gakstatter has been involved in the GPS/GNSS industry for more than 20 years. For 10 years, he held several product management positions in the GPS/GNSS industry, managing the development of several medium- and high-precision GNSS products along with associated data-collection and post-processing software. Since 2000, he's been a power user of GPS/GNSS technology as well as consulted with capital management companies; federal, state and local government agencies; and private companies on the application and/or development of GPS technology. Since 2006, he's been a contributor to GPS World magazine, serving as editor of the monthly Survey Scene newsletter until 2015, and as editor of Geospatial Solutions monthly newsletter for GPS World's sister site Geospatial Solutions, which focuses on GIS and geospatial technologies.

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