JoCoSAR Blog

Map Problem For Compass

By Brian O'Connor

Rectification (or how to use some visible landmarks to triangulate your own position)

You have a map and your compass, and you're "somewhere" out in the wilderness.
        After orienting your map to true north, you can identify Eagle Mountain and Fiddler Mountain "in the real world."
        The bearing of Eagle Mountain is 13 degrees.
  The bearing of Fiddler Mountain is 92 degrees.

Using the map below, locate your approximate position, and represent it in UTM UPS coordinates (you may want to print the map, though you can work off your computer display without marking on it (!) using "trial and error").

        You can click on the map to enlarge it and repeat for an even larger image.

 

Google Maps Nav Will Not Kill Standalone GPS

David Coursey | Thursday, October 29, 2009 7:00 AM PDT

Yesterday's introduction of the Google Maps Navigation service was seen by many as the death of standalone GPS. That isn't true and rumors of the death have been greatly exaggerated.

Consider the whole issue of smartphones-as-GPS devices.

Google's new navigation service is free and seems to provide all the features I'd want in a GPS, provided I have a data connection on the handset. That is a major limitation, as I will explain.

The Google solution also requires me to use a smartphone as a primary GPS and it may prove difficult to simultaneously use it as a phone.

It is certainly possible, as TomTom is doing with the iPhone, to load enough software to make the handset roughly comparable to a dedicated GPS. That means installing not only the GPS application, but also all the maps the user is going to need.

That works, but is expensive. TomTom wants $100 for its iPhone application and another $120 for its car kit, shown here. (Read our review).

Note that the built-in iPhone GPS is so lousy that TomTom builds another receiver into its car kit. The company admits that even with that second receiver the iPhone does not receive as well as a standalone GPS, which you can purchase for far less than the combined $220 cost of the TomTom iPhone solution. In addition, you still have the problem of using a single device as a phone and GPS simultaneously.

My guess is that, TomTom's car kit notwithstanding, most people will not mount their smartphone-as-GPS up on the dash or windshield, where the phone features might be inconvenient to use. Rather, I expect users will set the GPS/smartphone on the seat or into a shirt pocket.

It would surprise me if this form of navigation turns out to be any less distracting to the driver as texting or reading e-mail and certainly requires as much handling of the phone as placing or answering a non-hands-free call might.

For many drivers, use of a smartphone GPS while in motion will create safety issues that a properly mounted traditional standalone or in-vehicle GPS system does not. That is why I am still using my Garmin Nuvi when I drive--it is easy to see while I keep my eyes on the road.

My Garmin is also my hands-free device in the car. Yes, I have more than one in-the-ear Bluetooth device, but find them uncomfortable to wear. When the phone rings, I do not want to have to find one just to answer the call. The Garmin's speakerphone works fine.

When thinking about the Google nav service, the need for a data connection should not be underemphasized. That means there are many places where people need a GPS and the Google service will not be available to them.

Did I mention screen size? The standard GPS today has at least a 4.3-inch display and my next one will be 5-inches. The iPhone screen is 3.5-inches and the Droid's display is just a touch larger.

Where GPS screens are concerned, bigger is a definite plus.

As a search-and-rescue team member, I would much rather know my missing person has a handheld GPS and a pocketful of AA batteries than is relaying on a cell phone for navigation.

In the case of an iPhone, power can be a problem if you are keeping the screen lit for long periods so the GPS map is visible. Even if you have the proper topographic maps on your phone, it is not a good choice for navigation in the woods.

If all you are doing is some neighborhood Geocaching, a smartphone/GPS may be fine, but if your life may be at stake, please bring a compass and paper map as a backup to whatever electronic tool you have chosen.

For serious outdoor recreation, I do not see smartphones replacing handheld GPS devices for reasons too lengthy to fully explore here. In automobiles, I believe larger screens and features that match what Google is offering will keep the GPS market alive.

Finally, there are also people like me, who do not mind carrying the right device for the task. A smartphone can be an acceptable or even a good GPS, but in many circumstances it will not be the best choice.

Long live the standalone GPS!

David Coursey tweets as @techinciter and can be contacted via his Web site. He is certified as a SARTECH II by the National Association for Search and Rescue.

Are All GPSs Created Equal? No, But Should You Care?

 A good entry level (less expensive) GPS will tell you where you are and take you where you tell it to take you via a dizzying choice of data options (NAD 27, etc., etc...., so why pay more?  Is it just to get a color screen?  Well yes, you can get that for a bit more, but as the cost goes up you can get some pretty useful additional tools that might be worth what you pay for them— such as the capacity to install topo maps of Oregon, California, Washington, Nevada (on One DVD) or, in increments, the whole USA and beyond.  But are there significant performance differences?  During the CORSAR mock search last month, navigating by GPS under thick canopy (in addition to the rain) demonstrated some surprising differences among GPS units in their ability to track and lock satellites.  Some units not only took a lot longer to get the minimum 4 satellite lock, but some didn't get 4 or lost one at times in the irregular, forest terrain where a 3D lock was essential.

So here are the results from a simple head-to-head comparison of satellite acquisition for a Magellan 210 Explorist ($120-179), a Maellan 500 Explorist ($139-400) and a Garmin 60Cx ($257-350).  The GPS units were compared for how quickly and how many satellites they could lock indoors (a very dense "canopy"), in an open area on a hilltop, and under tree canopy.

Findings:  The Explorist 210 couldn't lock on any satellites indoors, after 3 minutes the Explorist 500 had 2 satellites and the Garmin had 7 locked.  In an open field the Explorist 210 locked 8 satellites in just under 2 minutes, the Explorist 500 in 90 seconds and the Garmin in 40 seconds.  Under a tree canopy the two Explorists took about a minute to lock 6 satellites while the Garmin had 9 locked instantaneously.

Summary: There is a difference in how quickly GPS units can tract satellites and how many. 

    

Caption:  Blue is the the Explorist 210, Red the Explorist 500 and green is the Garmin 60Cx: the first graph is the results indoors, the second is the open area and the third is under tree canopy. The green bars show up quicker and are stronger, i.e. the Garmin wins- with antennas, size does matter.

And then there is all that other nifty stuff to consider such as topo maps, color displays, etc..  So if you needed some justification for a new GPS, this information just might help.

How To Navigate Along a Contour Line on a TOPO Map

By Brian O'Connor

From time to time, we're asked to search along a contour line or two ("out" on one, "back" on the other).

That sounds easy enough, but it's a little more complicated than one might suspect.  For example, even the best GPS barometric altimeter (like the one in Garmin's 60CSx) is accurate only to ± 70 feet,  which is almost two 40-foot contour intervals.   And if you're on a gentle slope, and it takes 200 feet of horizontal travel to climb 40 vertical feet, you could be way, way off.  (The altitude computed by a standard GPS unit, which calculates elevation from satellite transmissions is even less accurate [± 150 feet].)

Then there's the problem of GPS lock.  If you're under dense cover and don't have 4 satellites locked up, you won't (by definition) have a 3D position fix (including altitude), and you may not even have enough satellites for a 2D fix.

What then?

Jim McNutt and I were talking about such things, and how a Type 15 compass might be used to follow a contour line using the compass's pendulum clinometer, and Jim suggested shooting what amounts to an elevation bearing in much the same way you'd shoot a regular compass bearing.

I thought about that, and did some tests, and here's what works:

1.  Ignore declination offset, and set the bezel to 360 degrees.

2.  Hold it like you were going to take a bearing (mirror open and on top, needle reflected, etc.).

3.  Rotate the compass and mirror 90 degrees (clockwise, if you're right handed) so that the sighting notch is 90 degrees from where it normally would be (at 3:00 o'clock, not 12:00 o'clock).

4.  Now — look at the inclinometer in the mirror, and align its line with the 90 degree line of your declination scale, using the mirror's sighting line just the way you'd shoot a bearing!  (You can ignore the needle and the lines etched in the capsule floor.)

5.  Find an object through the sighting notch, and you've just found your contour line (you can turn your body right or left, to find a convenient target, which you then walk to)!

Caption:  Notice that the compass is on it's right side; where the sighting notch is, and that the clinometer is aligned with 90 degrees on the declination scale.  (In this photo, the sighting line on the mirror is a little off-off center — it should pass directly over the needle's pivot point — because it was impossible to get the camera perfectly oriented.)

CORSAR: Communication Team

The critical link between the searchers in the field and incident command, where the search strategists are located, is the communications team.  During Sumer Ex we used the state SAR radio frequency for ICP communications with the field and FRS radios for communication among team members.  Having approximately 17 teams in the field (and the transporters that got them there) all using the same radio frequency kept the base communication team very busy (they also transcribe all messages received).  About 6 or 7 searchers were rigged with APRS (automatic position reporting system) vests that automatically send the GPS location of the person to command at the time interval set by command (shorter time when vehicles are used, longer for walkers, etc.).  The GPS location appears on the computer displaying a topo map of the area  allowing coms to track the trackers without voice communication while obtaining a permanent record of their position at 3 minute intervals.  Pretty neat!

These big exercises do expose communication problems that we can't recognize when we draw the plan up on paper.  Solutions to these problems come from working the system in a realistic setting and then finding the weak points in the plan.  We found several ways to improve our system validating the necessity of testing a system under rigorous field conditions- which we certainly did.

Some of the K-9 teams have APRS devices on their dogs that track the position of the dog and record it on a handheld unit the handler carries.  This allows for detailed analysis to the areas covered by the dog team that helps search planners know what areas have been well covered and which not.

    

      

Magnetic North... A Moving Target

CORSAR COUNTDOWN:  38 Days

We all know that your compass, without adjustment, points to magnetic north.  The magnetic north pole is where the earth's magnetic field points vertically downward. The Earth creates its own magnetic field from the electric currents created in the liquid iron-nickel core.  

But let's get practical:  If you want your compass to show you the direction of true north you need to adjust the declination of the compass.  As you know, magnetic north isn't fixed in place— records show that it has wandered around the Hudson Bay area over the last four hundred years as much as hundreds of miles.  Since the 1970's, the magnetic north pole has accelerated from less than 10 to more than 30 miles per year.  An animated video demonstration of this created by NOAA is impressive.

On an even more practical note: Bill Colisch, using professional surveying equipment, has determined that the magnetic declination at SAR house is 19 degrees east.

On the video below: The black line is where the declination equals zero and the direction of True North and Magnetic North are equal. 

Hat tip to Brian O'Connor

GPS Theory- How It Works

Last Saturday we had a great GPS training session with slides and a geocache exercise.  For those who, alas, missed it or want a review of some of the material presented, here is info adapted from the excellent site, "How Stuff Works".

The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails).  The U.S. military developed and implemented this satellite network as a military navigation system, but soon opened it up to everybody else.

Each of these 3,000- to 4,000-pound solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.

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Photo courtesy U.S. Department of Defense

Artist's concept of the GPS satellite constellation

A GPS receiver's job is to locate four or more of these satellites, figure out the distanc­e to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration. Trilateration in three-dimensional space can be a little tricky, so we'll start with an explanation of simple two-dimensional trilateration.

Compass Stuff

More about Compasses

We marvel that a compass needle will always point north no matter where we stand on the earth.  A compass is an amazingly simple and brilliant tool any minimalist will take great joy in using.  We know you must keep your compass level when taking or shooting a bearing which seems simple enough.  But ships and planes have problems keeping their compasses level which affects their utility.  To solve that problem gyroscopic compasses were developed. While we don't need gyroscopes for land navigation, the gyroscopic compass is yet another simple, by today's tech standards, solution to a critical navigation problem.  

Take the compass quiz offered by "How Stuff Works" to test your compass knowledge (the ten questions start easy and get harder).  If you ace the test, test the knowledge of your family and friends and see how they do (and how much you have learned).

Compass Quiz

Didn't know about gyroscopic compasses?  Here's what you need to know.

Gyroscopic Compasses

A magnetic compass has several problems when used on moving platforms like ships and airplanes. It must be level, and it tends to correct itself rather slowly when the platform turns. Because of this tendency, most ships and airplanes use gyroscopic compasses instead. 

A spinning gyroscope, if supported in a gimbaled frame and spun up, will maintain the direction it is pointing toward even if the frame moves or rotates. In a gyrocompass, this tendency is used to emulate a magnetic compass. At the start of the trip, the axis of the gyrocompass is pointed toward north using a magnetic compass as a reference. A motor inside the gyrocompass keeps the gyroscope spinning, so the gyrocompass will continue pointing toward north and will adjust itself swiftly and accurately even if the boat is in rough seas or the plane hits turbulence. Periodically, the gyrocompass is checked against the magnetic compass to correct any error it might pick up.  Pretty cool!

Adapted from "How Stuff Works"   

GPS Practice with a little Geocaching

Geocaching is an activity in which a small object is placed somewhere at large and others, using the UTM coordinates of the object, try to find it. It sounds easy, but you have to know a lot about how GPS units work, where the potential errors come from and how to work around those limitations to succeed in finding the object.   There are many great variations on geocaching games that include making the hunt international, but our geocache practice is done to cement our GPS skills- and have a little fun running around in the nav field.

http://www.geocaching.com/

Today 26 stalwart souls turned out for an in depth GPS course followed by an informal geocache competition in the SAR land navigation field.  Poker chips were positioned by GPS days in advance and the participants had to find and retrieve the chips assigned to them by GPS - the quickest to find their three chips won prizes.  Some chips had probably been moved by wind or beasts and others covered with leaves making the playing field arbitrarily uneven allowing us to debate whether the winners just had lucky chips or were really more skilled.  

Significantly, everyone present worked their GPS units like a pro and could quickly locate their chips UTM coordinates.  The second challenge, which proved the more difficult one, was to be able to find the small object - equivalent to an evidence search for a small object.  All chips were placed in plain view, but seeing the chip you were looking for proved the most difficult part of the exercise.  

Two out of the three geocache top finishers were new members- way to go!

Thanks to all who turned out who deserve a genuine, "Well done!"

For more about geocache, check here.

Datum Details

Devilish Details... how to adjust for NAD 83 on your NAD 27 map 

By Brian O'Connor, SR 83

In an earlier post, we discussed what a map's Datum was, why there any number of different Datums, and stressed that it's important for you to set your GPS Datum to the one that the cartographer who drew your map used.  Otherwise you'd not be able to locate your position on your map with the optimal precision.

We also mentioned that we of SAR use the Datum NAD27 CONUS, but that there was a more current Datum — NAD83 — that was becoming increasingly popular and used on newer maps.

This post will illustrate that for a given area, a map constructed with one Datum will not be congruent with a map constructed with another Datum  — in essence, one set of Easting and Northing values will indicate different locations on maps of the same area, but constructed using different Datums (in this case, NAD27 vs NAD83).

To avoid the confusion and inevitable errors of having two different Datum systems representing the same area, and because there are millions of 1:24,000 NAD27 topo maps already in use that it'd be impossible to withdraw (irrespective of costs), the USGS is (at least for the time being) continuing to print maps using NAD27, even though NAD27 is obsolescent.

But mindful that NAD83 is becoming more popular, the USGS has included  little "+" marks near each corner of the NAD27 map to indicate the NAD83 offset.