Caching with a Cell Phone and Compass

[oakenwood]

1. Introduction & Equipment

 

Maybe you'd like to add some challenge to your caching. Maybe you'd like to try something new and learn a few things. Maybe you want to get into geocaching on a small budget. Many cell phones will give you GPS coordinates. You can buy a new GPS-enabled phone for under $40. You might even get someone's "old" phone for free. There are navigation and geocaching apps available for these phones, but here's how to do the calculations yourself and navigate to the cache with just a compass. If you have a "regular" GPSr, you can use it for the coordinates and compass and ignore the navigation features.

 

You'll need four things: 1) the phone or GPSr; 2) a scientific calculator; 3) a good compass; and 4) paper and pencil for calculations. Okay, that last one is actually two things. The good news is, you won't need a map. There's no rule against using one, however. The calculator must be able to perform square root and arccosine calculations. The arccosine button will have a label that look like "cos-1", or possibly "acos" or "arccos". It doesn't say "cosh". That's a different function. The compass should be the kind with an adjustable bezel. You can find these at sporting goods stores for about ten dollars.

 

The paper and pencil part is simple, but it helps if you have a form set up before you start. Here's a picture of the form I use. It has some reminders built in to help prevent mistakes, and a mini-map to help show what direction the cache is in. The top form is filled out; the destination is my now-archived puzzle cache "Randolph and Parklawn".

 

2. Before You Start

 

You need to get a couple of numbers before you start. If you're using a magnetic compass, you'll need to find your magnetic declination. If you are using a GPSr for your compass, you can ignore this, because it points to True North. To find your declination, go here and click on your location, or here and enter your coordinates. Round the result to the nearest degree. If the declination is west, you'll add it to your bearing. If it's east, you'll subtract it from your bearing.

 

You'll also need conversion factors to convert GPS coordinates to distances. The conversion factor for latitude is 1.855. It's the same everywhere. The conversion factor for longitude is not. Here's how you find it: take a GPS reading at your location. Go here and enter your coordinates as "source". (Carefully follow the formats they give you.) Enter the same coordinates as "destination", but add one minute to the longitude. Under "units for results", select km, and click "send query". The result is your conversion factor for longitude. If you stray more than 50 miles north or south, get a new conversion factor for that longitude.

 

3. Calculations

 

Now you can head out to the cache area. Pick a location to begin and get your coordinates. Subtract the starting coordinates from the destination coordinates and apply the conversion factors. A positive number for the latitude means the cache is north of you, and a positive longitude number means the cache is to your west. You can use the mini-map on the form to plot its general direction. It doesn't have to be exact. (See the example form in the picture.)

 

Now you can calculate the distance and bearing to the cache. First we'll do distance. It's pretty simple. Take each of the distances from the last paragraph and square them, then add the results together. Press the square root key, and there's your distance to the cache.

 

Calculating the bearing is only a little harder. Look at your mini-map. Draw a triangle like this: draw a line from the center of the map straight to the cache, then turn left and draw a line perpendicular to the axis it's headed towards, then when you reach the axis, turn left again and draw the line back to the center. Each side of the triangle has a name. The long side of the triangle is called the "hypotenuse". Its length is the distance to the cache, which you just calculated. The other two sides are called the "adjacent leg" and the "opposite leg". The adjacent leg is, naturally, the one adjacent to you.

 

To calculate your bearing, divide the length of the adjacent leg by the length of the hypotenuse (don't forget to press the equals key), then press the arccosine key. This number is called the "theta". It should be between 0 and 90. If it's outside this range, something's wrong.

 

Your adjacent leg will always point directly north, south, east, or west. Look at your mini-map and it'll show you. North equals 0, east equals 90, south equals 180, and west equals 270. To get your bearing to the cache, you add your theta to the adjacent leg. If you're using a magnetic compass, you'll apply the magnetic declination to this number.

 

I recommend practicing the calculations at home before heading out. A little mistake can send you way off course, and it's common to make mistakes at first. The good news is, if you make a mistake in the field, you can just get a fresh reading and do a new calculation. If you completely blow it, at least you have your GPS coordinates when you call for rescue. You also might begin with a trial run by going after a cache whose location you already know.

 

4. Getting to the cache

 

Plenty of websites are available that show you how to navigate using distance and bearing. I'll briefly cover it here.

 

Take your compass and turn the bezel until your bearing number lines up with the big arrow on the outside of the compass (the "direction-of-travel arrow" or "DOT arrow"). There's another arrow inside the bezel (the "orienting arrow"). Line up the north pole of the compass needle with the orienting arrow, and the DOT arrow will point to the cache. Be careful how you hold the compass; if your fingers are too close to the needle, they can affect its direction and lead you off-course.

 

Keep the needle lined up with the orienting arrow as you walk to the cache. It's rarely possible to follow a perfectly straight line to the cache, so you'll have to estimate how far off you've drifted and make an adjustment when you've walked the full distance.

 

You can estimate your distance by counting your strides as you walk. You just need to know how many strides you take for every hundred meters. (A stride equals two steps.) An easy way to find out is to go to a football field. The distance from one goal line to the back of the opposite end zone is about 100.5 meters. Start by stepping forward with your right foot and count each time your left foot lands. Walk along the sideline so you don't wander back and forth. Take a few counts and average them.

 

The terrain on the way to the cache will affect your stride count. Walking uphill or over rough terrain will shorten your stride and increase your stride count. Zig-zagging will also increase it. Walking downhill will lengthen your stride and reduce your stride count. With experience, you'll get a good idea of how much to adjust your stride count for terrain. For a beginner, the rule of thumb is to add ten percent.

 

5. Conclusion

 

I've simplified things, left out some minor details and mathematical terms, and tried to keep the language brief, clear and simple. Consider this to be "version 0.9 beta". I welcome any questions, comments, or advice. I'm eager to see what you all think. In particular, I'd like feedback from anyone using this method to find a cache.

[+Team Wolf Clan]

1. Introduction & Equipment

 

Maybe you'd like to add some challenge to your caching. Maybe you'd like to try something new and learn a few things. Maybe you want to get into geocaching on a small budget. Many cell phones will give you GPS coordinates. You can buy a new GPS-enabled phone for under $40. You might even get someone's "old" phone for free. There are navigation and geocaching apps available for these phones, but here's how to do the calculations yourself and navigate to the cache with just a compass. If you have a "regular" GPSr, you can use it for the coordinates and compass and ignore the navigation features.

 

You'll need four things: 1) the phone or GPSr; 2) a scientific calculator; 3) a good compass; and 4) paper and pencil for calculations. Okay, that last one is actually two things. The good news is, you won't need a map. There's no rule against using one, however. The calculator must be able to perform square root and arccosine calculations. The arccosine button will have a label that look like "cos-1", or possibly "acos" or "arccos". It doesn't say "cosh". That's a different function. The compass should be the kind with an adjustable bezel. You can find these at sporting goods stores for about ten dollars.

 

The paper and pencil part is simple, but it helps if you have a form set up before you start. Here's a picture of the form I use. It has some reminders built in to help prevent mistakes, and a mini-map to help show what direction the cache is in. The top form is filled out; the destination is my now-archived puzzle cache "Randolph and Parklawn".

 

2. Before You Start

 

You need to get a couple of numbers before you start. If you're using a magnetic compass, you'll need to find your magnetic declination. If you are using a GPSr for your compass, you can ignore this, because it points to True North. To find your declination, go here and click on your location, or here and enter your coordinates. Round the result to the nearest degree. If the declination is west, you'll add it to your bearing. If it's east, you'll subtract it from your bearing.

 

You'll also need conversion factors to convert GPS coordinates to distances. The conversion factor for latitude is 1.855. It's the same everywhere. The conversion factor for longitude is not. Here's how you find it: take a GPS reading at your location. Go here and enter your coordinates as "source". (Carefully follow the formats they give you.) Enter the same coordinates as "destination", but add one minute to the longitude. Under "units for results", select km, and click "send query". The result is your conversion factor for longitude. If you stray more than 50 miles north or south, get a new conversion factor for that longitude.

 

3. Calculations

 

Now you can head out to the cache area. Pick a location to begin and get your coordinates. Subtract the starting coordinates from the destination coordinates and apply the conversion factors. A positive number for the latitude means the cache is north of you, and a positive longitude number means the cache is to your west. You can use the mini-map on the form to plot its general direction. It doesn't have to be exact. (See the example form in the picture.)

 

Now you can calculate the distance and bearing to the cache. First we'll do distance. It's pretty simple. Take each of the distances from the last paragraph and square them, then add the results together. Press the square root key, and there's your distance to the cache.

 

Calculating the bearing is only a little harder. Look at your mini-map. Draw a triangle like this: draw a line from the center of the map straight to the cache, then turn left and draw a line perpendicular to the axis it's headed towards, then when you reach the axis, turn left again and draw the line back to the center. Each side of the triangle has a name. The long side of the triangle is called the "hypotenuse". Its length is the distance to the cache, which you just calculated. The other two sides are called the "adjacent leg" and the "opposite leg". The adjacent leg is, naturally, the one adjacent to you.

 

To calculate your bearing, divide the length of the adjacent leg by the length of the hypotenuse (don't forget to press the equals key), then press the arccosine key. This number is called the "theta". It should be between 0 and 90. If it's outside this range, something's wrong.

 

Your adjacent leg will always point directly north, south, east, or west. Look at your mini-map and it'll show you. North equals 0, east equals 90, south equals 180, and west equals 270. To get your bearing to the cache, you add your theta to the adjacent leg. If you're using a magnetic compass, you'll apply the magnetic declination to this number.

 

I recommend practicing the calculations at home before heading out. A little mistake can send you way off course, and it's common to make mistakes at first. The good news is, if you make a mistake in the field, you can just get a fresh reading and do a new calculation. If you completely blow it, at least you have your GPS coordinates when you call for rescue. You also might begin with a trial run by going after a cache whose location you already know.

 

4. Getting to the cache

 

Plenty of websites are available that show you how to navigate using distance and bearing. I'll briefly cover it here.

 

Take your compass and turn the bezel until your bearing number lines up with the big arrow on the outside of the compass (the "direction-of-travel arrow" or "DOT arrow"). There's another arrow inside the bezel (the "orienting arrow"). Line up the north pole of the compass needle with the orienting arrow, and the DOT arrow will point to the cache. Be careful how you hold the compass; if your fingers are too close to the needle, they can affect its direction and lead you off-course.

 

Keep the needle lined up with the orienting arrow as you walk to the cache. It's rarely possible to follow a perfectly straight line to the cache, so you'll have to estimate how far off you've drifted and make an adjustment when you've walked the full distance.

 

You can estimate your distance by counting your strides as you walk. You just need to know how many strides you take for every hundred meters. (A stride equals two steps.) An easy way to find out is to go to a football field. The distance from one goal line to the back of the opposite end zone is about 100.5 meters. Start by stepping forward with your right foot and count each time your left foot lands. Walk along the sideline so you don't wander back and forth. Take a few counts and average them.

 

The terrain on the way to the cache will affect your stride count. Walking uphill or over rough terrain will shorten your stride and increase your stride count. Zig-zagging will also increase it. Walking downhill will lengthen your stride and reduce your stride count. With experience, you'll get a good idea of how much to adjust your stride count for terrain. For a beginner, the rule of thumb is to add ten percent.

 

5. Conclusion

 

I've simplified things, left out some minor details and mathematical terms, and tried to keep the language brief, clear and simple. Consider this to be "version 0.9 beta". I welcome any questions, comments, or advice. I'm eager to see what you all think. In particular, I'd like feedback from anyone using this method to find a cache.

 

 

This should be made a sticky!!!!

[oakenwood]

This should be made a sticky!!!!

Thanks for the response.

 

This is a method one can use any time you have two points and need to figure the distance and bearing between them.

 

I'm thinking about turning this concept into a puzzle cache, maybe by reversing the process. That is, I give you distance and bearing to the cache from some point where you couldn't walk to the cache (across a river, highway, or other barrier) and you would have to calculate the cache coordinates.

[+Cardinal Red]

This should be made a sticky!!!!

Thanks for the response.

 

This is a method one can use any time you have two points and need to figure the distance and bearing between them.

 

I'm thinking about turning this concept into a puzzle cache, maybe by reversing the process. That is, I give you distance and bearing to the cache from some point where you couldn't walk to the cache (across a river, highway, or other barrier) and you would have to calculate the cache coordinates.

 

You just created a puzzle perfect for the PROJECT WAYPOINT feature built into my GPS. And it's been done before.

[oakenwood]

You just created a puzzle perfect for the PROJECT WAYPOINT feature built into my GPS. And it's been done before.

How about if I give you the starting coordinates and then the latitude or longitude of the cache and the bearing to it? You have to figure out the point of intersection.

 

I've been following this thread, which focuses on distance calculations, so I think I'll try bearing instead.