iGeoCacher - The New HSI Compass - Which way *is* it anyway?

Verson 1.11 Update Note:

There is now an "Auto" mode in addition to the Line1/Line2 discussed below. The net effect is as if you pressed Line1 then moved at least the distance of the current GPS accuracy in one direction and pressed Line2. Basically, auto mode continuously samples position and when you've moved at least the accuracy distance from the previous point it updates the line of position automatically. Try it out while moving in your car with the scale rotation (new button not shown in older graphics below) set to track "Line" and you will have an automatic in-car compass ;-)

Some Background

One of the first features that everyone asked for when iGeocacher 1.0 came out was some sort of compass display. In truth, I wanted one too. But there were a couple of reasons that didn't make it into the first release. First, until Apple announced it we didn't know there was going to be a GPS in the new 3G phone. That was a late piece of information. Second, learning to program the iPhone isn't all that hard but there is a HUGE amount of material to get acquainted with. I just hadn't had the time to get familiar with the graphics API's that a compass would require. Finally, having experienced the compass on my Garmin eTrex, I wanted something that would be less likely to mislead casual geocachers who might not realize the limitations that a GPS has in determining actual direction. GPS's measure position. They can calculate direction from CHANGES in position assuming that the changes happen in a straight line from one sample to the next. The accuracy of determining direction is also heavily dependent upon the overall accuracy of the position reading at the moment (how well your GPS is determining current position) and the distance BETWEEN the relative points.

The Problem in Practice

The problem with most GPS directional displays is that in the last stage of the hunt when the casual geocacher is getting close to the target, they are excited and while up to this point they've been walking a more or less straight track toward the target from their starting point, now that they are near they are all over the spot walking a few steps this way then a few steps that way, vainly trying to follow the arrow on the device which by now has almost no chance of pointing in the right direction based upon their erratic movements. This almost always frustrates my daughters and I have to remind them that close in they are genarally better off just walking a few steps, seeing if the distance gets larger or smaller and then either continuing or back tracking until they've minimized the distance as best they can along a line. Then walk perpendicular to that line and repeat the process and IGNORE the Garmin eTrex compass readout just using the distance number.

The Promise

So I promised myself that when I did a compass display I wanted to do it right. By that I mean do it in a way that would be easy to use and positively orient the user with minimal effort. So I started by merely calculating the bearing from present position to the target. That is a straightforward piece of spherical trigonometry. The original display merely displayed that number. My hope was that any serious cacher would take a magnetic compass along with them and simply read that bearing and use an actual magnetic compass. Now I'm going to neglect variation discussions at this point because it isn't all that big a factor in most hunt endgames. Just knowing the basic direction is the important part and the average cacher can do that with a magnetic compass FAR more accurately than the result they'll achieve with the typical GPS compass display. At least this has been my observation with numerous young people in various land navigation scenarios including geocaching. I've also done a lot of Search and Rescue work and ground team training with the Civil Air Patrol and I've seen the mistakes folks make first hand.

Well, while as a pilot I have a pretty good sense of bearing to direction from hundreds of hours in the cockpit, most folks don't and so when you tell them it's a bearing of 180 degress, they don't immediately think "South." So you next take the bearing number and translate it into a graphic pointer on a compass scale. But that scale doesn't help much because it is merely "painted" on the face of the GPS (in this case the iPhone/iPod" which you could be holding facing any direction.) There is no magnetic direction capability to determine direction in the device (wouldn't THAT have been sweet!)

So now they know that 180 degrees means south but they haven't got a clue (especially on the overcast days when you can't see the sun) which way south REALLY is if they don't have a true compass or some other reference. What you need in the field is orientation that you can easily reference visually and translate into your immediate surroundings. Now as I said, deternining the bearing between a couple of GPS coordinates is not hard mathematically and that's exactly what your Garmin is trying to do for you. The problem is, the Garmin is controlling "when" to take the samples. It's doing a running sample at some rate the designers thought optimal for the general case of someone walking in the woods and they might even be doing some clever statistical averaging to try and minimize the error. The accuracy of direction gets better the farther apart the samples are in actual position but unless you want to spend a lot of time sprinting in the woods there is only so far you can move between the times that the manufacturer has chosen for his samples. The problem is, you can't control WHEN and WHERE the reference samples are taken with a typical GPS. Enter the iGeocacher HSI method. It's a simple concept really. I just let you choose WHEN and WHERE to take the direction determining samples. It produces a better result on average because you have complete control over the samples used to determining direction. You can choose two points in your vicinity that are easy visual landmarks and walk to one, take a position then leisurely walk to the other by any route the terrain affords with safety and take the other point when you are there and the position is accurate. With the second point iGeocaher dutifully does the math and shows you the accurate direction between the first point and the second point.

Now you have an arrow on the display that points to the cache target AND you have an arrow that clearly shows the line of bearing between the two prominent landmarks in your vicinity. Simply orient the unit to align it with your two reference points and the target arrow points accurately to your target, rain, shine, day, night, it's all good.

I didn't realize how well this actually would work out until I did the first field trials. My second cache of the day was located at a nice local park with some relatively open areas. I had the cache dialed in and I knew it was somewhere more or less north within a couple of hundred meters. We got out of the car and there was a nice sign at the entrance to one of the fields. Starting there I picked out a small tree in the distance that stood out as my walking target. It happend to be at what looked like about a 30 degree or so angle to where I thought the cache might be but that isn't really important. The main thing was that it stood out as a clear landmark that I could see from anywhere in the immediate vicinity. I pressed the "line 1" button to take the first sample position and started walking toward the tree. The tree was probably a hundred or more meters away so I knew I didn't have to go all the way there. I just needed something I could see clearly. After about half the distance was covered I hit "line 2" which recorded the second point. Instantly I got the yellow directional arrow showing me the bearing between the entrace sign pole and where I stood on a line walking to the tree. Now if there had been a pond or landscaping that I would have had to walk around, I could have and it would have made no difference because I wasn't "sampling" position except when I command it by pressing the sample button. As it was, I didn't have to in this case but I wanted to make it clear that you don't have to get to the second point in a straight line. However, in this case there was an unanticipated benefit in having done so. By definition the first point was behind me and the tree was still dead ahead on the same line. WIthout even having to look back all I had to do to line things up was simply point the arrow that had just appeared on the display directly at the tree! The red target arrow then pointed directly at the cache. Now I could also read out the actual true north directions and bearing as well because this effectively oriented the iPhone in real space but I hardly noticed. We just visually picked noted the direction the red arrow pointed and took off watching the distance display click down.

But it gets better. We had to bushwhack a bit to enter some wooded overgrowth at the edge of the park and so lost sight of our original references. But there was a worn trail on the forest floor. Once again, I pressed "line 1" walked down the trail to another good spot and pressed "line 2" The yellow arrow shifted to line up with the trail which made it a piece of cake to follow the red arrow right to the cache. Endgame. It was almost too easy.

Now that is probably not easy to follow so I did this little pictorial demo in my front yard for you a couple of hours ago before the sun set. I took a red Igloo cooler that we have to serve as my "cache" and put it in the front yard about 5-10 meters or so back from the street which curves past the front yard in a more or less East-West direction. Here' a shot from the Google map display of the street. The cache is just about where the blue GPS dot is.

Road View Satellite view

Above you see the cache and my caching buddy.

To simulate this and set up the demo I need the cache coordinates so standing next to it I let the GPS settle and...

Press the mark button, go to the WPT screen and fill in "Big Red" as the waypoint name and add it to group 5.
Now to see it in my caches display for selection I have to make sure group 5 is "on". And there it is, 1.8 meters away to the east (g).

So to set this up I now just select the "Big Red" waypoint which loads it into the GPS just like any other geocache.

I walked 100 or so meters West on the road and took this shot facing back East. Notice the mail box pole and the telephone pole in the distance. Remember those. See the cache and my buddy in the distance.

Notice "buddy" decided to sit this one out.

So here I'm starting out, I line up the mailbox pole with the power pole in the distance. Note the cache ahead and slightly to the right. Notice the red arrow slightly to the right. The cache is truly "East" of me but in a real scenario I wouldn't yet know which way "East" was except that I'm standing on my own street.
So I press the "line 1" button and start walking down the street toward the power pole. Notice that there is no yellow arrow yet but the button says "line 2" indicating that it is waiting for me to mark the second point in determining the line of bearing.
I'm walking down the road West to East more or less  
Walking Eastward, north of cache At this point about half way to the power pole I hit "line 2" which gives me an arrow showing that I'm walking East. In the real scenario this would be the first time I knew that. I'm actually standing about six meters North of the cache according to the display. Notice that the button has toggled back to "line 1" ready for another two point "fix" any time I need one as I progress.
Continuing Eastward, NE of cache You can see how the red arrow "tracks" my relative postion as I continue on my East West Line. The yellow arrow stays locked on the bearing line I established with the two points until I elect to establish another. If I point the yellow arrow at the power pole which was my second reference point, the red arrow points right to the cache.


I recommend that you get out in the yard or a park and play with this a bit to get the idea before you go into the field. It is amazing how fast you can nail your direction in the field in unfamiliar territory. For best results in terms of direction you want to chose points that are at least as far apart as two or three times the accuracy reading in the lower right corner. but practically just do the best you can to mark out a couple of points that you can easily see and reach to take the line 1 and line 2 position readings. At that point, the yellow arrow is aligned in the direction of that line from point 1 to point 2 and the red arrow points in the direction of the cache. If you are standing between the two points, just point the yellow arrow at the second point. If you aren't on the line between the two points just orient the unit so that it parallels that line and you'll have the same result.

Now reading this it may sound more complicated than it is and the pictures don't really do justice. Just get out and experiment with it and I think you'll love it as I do. It's quick, easy, and leaves no ambiguity.

Finally, I call this a Horizontal Situation Indicator after the instrument of the same name that most pilots LOVE to have in their cockpit. The reason they love it is because it gives them a LOT of information about their orientation in the air in one place. With this display you have a neat and compact display of the lats and lons, the direction of the cache and a line which helps you orient the unit with respect to "real" north using two reference points which YOU choose at your convenience for ease of reference and accuracy. They aren't chosen for you by some programmer making assumptions about how you are walking in the woods.

But having said all of that, when it comes down to the end game, pay ever more attention to the distance display and less to the direction. Any cacher knows it's the last 30 feet that are the hardest because then distances and directions are at the accuracy limit of the device. Keep your eyes open and don't get so focused on the display that you step into a hole and sprain an ankle like I've almost done. I hope you enjoy the HSI display. If you have ideas about improving it, pass them on. I've put a lot of thought into it both from my experience as a cacher but also as a SAR Mission Pilot. Together with the other features, I'm pretty confident to say that a better single unit package for geocaching doesn't exist in any other mobile platform. But beauty is always in the eye of the beholder. I love my rig. I guess we all do.

Enjoy, be safe and good caching!