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Nov. 16 Webinar Q&A Follow up: A Buyer's Guide to GPS/GIS Mapping Equipment Ver. 2.0

November 22, 2010 By: Eric Gakstatter

On Nov. 16th, I conducted a webinar entitled "A Buyer's Guide to GPS/GIS Mapping Equipment Ver. 2.0". Before and during the webinar, there were many questions. I addressed some of them in the webinar, but as customary, I follow up with a newsletter addressing the ones I didn't.

If you weren't able to attend and want to view the 60-minute webinar, you can view the archive by clicking here.

Typically, I present the poll results in the newsletter as well, but I'm to do that differently this week. I'm writing an article for my GPS World Survey Scene newsletter that's going to summarize the polls from last week's webinar as well as a few others. I think you'll be interested in reading about what your colleagues' thoughts are.

In the following Q&A, I've tried to insert as many link as I can to give you some good reference material.Without further ado, following are the questions from last week's webinar:

Question #1: Are there any receivers that are yet set to acquire data from more than two constellations?

Gakstatter: Technically, yes, but probably not what you are referring to. In the context of mapping-grade GNSS receivers (not cm-level), there are several receivers on the market (Trimble, Ashtech, Topcon, Leica) that utilize GPS, GLONASS and SBAS constellations. You probably didn’t expect me to mentioned SBAS, but each of the SBAS is a regional constellation, albeit very small (typically two geosynchronous satellites). Some receivers use the ranging data from the GEOs to add another observable to the position solution (in addition to traditional the SBAS info).

However, SBAS is a regional system, as well as Japan’s QZSS. There’s not anything in terms of a third global constellation. Galileo (Europe) is getting closer and Compass (China) is heating up. It’s pretty much the same story for survey-grade receivers, although GLONASS is much more prevalent in those models. Most GNSS receiver manufacturer's track the Galileo test satellites and some innovative receiver manufacturers have already announced they are tracking the Chinese Compass test satellite, but the reality is that neither of these constellations are useful for production surveying or mapping work at this time.

Question #2: We are using Hemisphere A221 with Omnistar XP licenses, are their any ways to improve it use in canopy/forest area's? Either by adding an extra antenna, or using a different GPS device.

Gakstatter:

Under tree canopy, you have to temper your GPS expectations. With OmniSTAR, it complicates the issue even further, because not only do you need to worry about tracking GPS satellites, but also the OmniSTAR satellite. Tracking the OmniSTAR satellite under tree canopy is probably the biggest challenge. Whereas you have many potential GPS satellites to choose from (eg. 10 in view, but 6 are enough), OmniSTAR is a hit-or-miss proposition. You have only one (maybe two) OmniSTAR satellites in view. If it's not in view for the receiver to track, you are stuck.

I'd first talk to Hemisphere and/or OmniSTAR and see what they suggest.

However, my feeling is that you may be pushing the technology beyond its capabilities, especially if you are expecting OmniSTAR XP (+/- 15cm) or HP (+/- 10cm) results under tree canopy. That's not going to happen. Even with OmniSTAR's VBS service (sub-meter), you're not going to get sub-meter accuracy under tree canopy.

I think you have to adjust your expectations regarding accuracy/performance under tree canopy. If you give me an idea what you want to achieve, I'll try to give you some idea of what technologies might work.

Question #3: What's the difference between GIS vs GIS data collection software?

Gakstatter: GIS is the Geographic Information System itself such as ArcView, AutoCAD Map, and GeoMedia. GIS data collection software is a tool used to collect data that's to be used in a GIS. Examples of GPS data collection software are ArcPad, Pocket GIS, Field CE GIS, and DigiTerra Explorer.

Question #4: In general, what is the relationship from the horizontal to vertical measurements? (i.e. subfoot horizontal = _____ for vertical)

Gakstatter: On higher-precision equipment, such as sub-meter, sub-foot, and cm-level receivers, generally your vertical accuracy is going to be 1.5-2 times worse than horizontal accuracy. It could be worse though if the VDOP (Vertical Dilution of Precision) is particularly poor compared to the HDOP (Horizontal Dilution of Precision).

On lower quality GPS equipment, vertical accuracy can be tens of meters in error.

Also, note that when working with vertical measurements, you must consider the ellipsoid height with respect to your local vertical datum and how accurately your local vertical datum models the geoid separation. Although dated, a good technical reference is this paper published by the U.S. National Geodetic Survey many years ago.

Question #5: When using sensor input, how do you convert from magnetic direction to grid direction and from slope distance to grid distance?

Gakstatter: This is going to be a function of the software you use. From magnetic to true north (assuming that's what you meant by "grid"), look at this website. If you're trying to convert from magnetic to other than true north, then I assume you have a model that defines the grid reference.

Regarding slope distance converting to horizontal distance, here is a good reference website.

Question #6: Tree canopy and buildings can give similar results.

Gakstatter: Good point. I agree it’s similar, but buildings that create an "urban canyon" environment in a downtown area with multi-story buildings, are much more absolute in terms of denying GPS service to users. In my opinion, tree canopy is a more forgiving. In other words, in some urban canyons evironments, your GPS receiver might be unable to view enough satellites 75% of the time, whereas under heavy tree canopy, that might occur only 40% of the time.

Question #7: What is the difference between SBAS, WAAS and DGPS?

Gakstatter: First of all, WAAS is an SBAS. SBAS (Satellite-Based Augmentation System) is a regional service that broadcasts integrity and correctors to GPS receivers. At this time, there are three operational SBAS in the world. WAAS covers North America, EGNOS covers Western Europe and North Africa, MSAS covers Japan. Both India (GAGAN) and Russian (SDCM) are planning their own SBAS.

DGPS is an acronym for Differential GPS. DGPS is a method of refining, in real-time, GPS accuracy to minimize or eliminate several sources of error. This requires at least one stationary GPS reference station (to generate the DGPS correction) and some method of wireless communications (to send the correction data to the GPS receiver in the field). Some countries have installed a DGPS infrastructure in order to provide DGPS corrections over a wide area. Over 42 countries have installed some sort of DGPS infrastructure. DGPS is used widely for marine navigation near ocean ports and in major rivers.

The method of transmitting DGPS corrections to users in the field varies considerably. One common method is using the 283-325KHz beacon frequencies used by the marine industry. DGPS corrections on these frequencies are broadcast free of charge.

Question #8: With real-time correction, how do you know what your accuracy is?

Gakstatter: Generally speaking, you don't. Of course, you can monitor the key indicators of position quality (RMS precision, clear view of the sky, low PDOP, 6+ satellites being used) and the chances are that your receiver will perform at the accuracy level the manufacturer states. The only true test, using real-time or post-processing, is to collect data on a point with known coordinates that are referenced to a known datum. Then, you can compare the results of your GPS receiver to the known coordinates. I encourage everyone with a new GPS receiver to do this exercise. It's a very good method of understanding the expectations of your GPS receiver. Even more so, you can introduce different obstacles (your body, a metal frying pan, a tree limb) to better understand how your GPS receiver performs.

Question #9: In my area, we are tethered to AT&T for real time pricessing to a virtual base. Any remedies when AT&T service is not available for real time processing?

Gakstatter: Good question. I use the same AT&T service to access RTK Networks. There are two solutions I can think of:

A. You can use UHF/VHF radios in addition to the AT&T to extend your RTK Network range. Here's how.

You will need three components:

GNSS Internet Radio (free software)
Notebook computer running Windows and is connected to the internet (WiFi or mobile phone network)
UHF/VHF base transmitter

The UHF/VHF base and rover radios are the same radios that have been used for RTK for years.

You could set the notebook computer and UHF/VHF up in a truck on the edge of the AT&T service area and then broadcast RTK corrections to the RTK rover (needs a UHF/VHF receiver). Depending on where you park the truck (considering the surrounding terrain) and how high you extend the UHF/VHF base radio antenna, you could broadcast RTK corrections another 10+ miles.

B. Instead of using a notebook computer, GNSS Internet radio software and UHF/VHF base radio, you could purchase something like the RTK Bridge.

Question #10: Mission planning software. Do you have a link available for a site you recommend?

Gakstatter: There are several off-line mission planning software packages from various manufacturers, but I prefer the online software because you don't have to worry about updating the almanac. Two online GPS mission planning softwares are from Ashtech and NavCom. I prefer the Ashtech software because it also supports GLONASS and SBAS in the software.

Question #11: What about the issue related with the real time dissemination/availability of collected data, for instance surveys in disaster scenarios?

Gakstatter: I’m not sure I understand the question correctly, so I'll answer it a couple of different ways.

If I read your question at face value, then you are asking about how to dessiminate GIS data after you’ve collected it. Internet (email, FTP) is certainly #1 (whether by LAN connection or mobile phone network. You could use a web interface to serve the data up to the public (or whomever). Recently, Esri has offered disaster response services for the earthquakes in Haiti, Chile and New Zealand.

Courtesy: Esri

In terms of DGPS infrastructure, that's an issue for hurricane-raveged areas where the DGPS infrastructure (broadcasting tower, communications, power) is susceptible to damage or outtages that could disrupt DGPS services.

In that case, an SBAS (WAAS/EGNOS/MSAS) could be used to provide GPS corrections.

Question #12: Disadvantages of using GLONASS for real-time sub-meter mapping?

Gakstatter: The obvious disadvantage is that no corrections are broadcast by WAAS or MSAS for GLONASS satellites. They are broadcast by EGNOS. Secondly, most DGPS infrastructure aren't broadcasting GLONASS corrections. However, even without corrections, there is some value. Some manufacturers have developed algorithms to utilitize GLONASS even without corrections being broadcast by the SBAS/DGPS infrastructure. Depending on how the manufacturer implements it, it could improve positioning under tree canopy as GLONASS adds several more satellite signals that could be useful in that environment.

Question #13: Using a laser range finder for data collection, how does this help in navigating back to the collected points? What process should we adapt for the same?

Gakstatter: Whether a position is collected via GPS or laser rangefinder won’t affect your ability to navigate back to the point as long as you can relate the coordinates detemined by the laser rangefinder to a defined coordinate system.

For example, if I’m mapping utility poles using GPS and a laser rangefinder, the laser rangefinder measurement is going to be referenced to a GPS position. Therefore, the resulting coordinate from the laser is “GPS-friendly” and you’ll easily be able to navigate back to the pole.

In the not-so-easy example, you are using only a laser rangefinder (no GPS) and typing in a arbitrary reference coordinate (eg, N: 10,000, E: 10,000) as your reference point. If you want to navigate back to it using GPS, you need a way to relate your arbitrary coordinate to something your GPS will understand (UTM coordinates, geodetic coordinates, state plane coordinates, etc.).

Aiming a laser rangefinder (Laser Technology, Inc)

Question #14: How can I measure the different accuracies of different GPS models? How much accuracy do we need for GIS mapping?

Gakstatter: The first thing to check is the manufacturer's datasheet for different GPS models. That will give you a good idea where to start. Remember, that most GS accuracies on data sheets are expressed in horizontal RMS (63-68% confidence), so it's not a guarantee of accuracy. Also, remember to try-before-you-buy. Have a dealer or colleague lend you a GPS unit you are interested in so you can try it in the environment you'll be using it in.

GIS Accuracy? GIS isn't about accuracy. In one example, a useful GIS can be one that's only accurate to 100 meters (eg. very large scale map database), or in other example may require the GIS to be accurate to within 5cm (eg. parcel database). It really depends on what the GIS is going to be used for as well as your budget. In some cases, you may want 5cm accuracy, but you can only afford one meter accuracy.

Thanks, and see you next week.

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