Projecting waypoint

[+imajeep]

I'm doing a two-stage multi, where the second stage is a projection from the first. I find the first with no problem, but not the second. Others are finding it, so I know its there.

 

Here's my question: I'm using a Garmin 60 CSx. When it projects a waypoint, it does so using trig calculations, right? In other words, the unit's compass isn't used to project the way point, right?

 

Thanks

[+Sputnik 57]

The cache owner should tell you whether the projection is in "true" degrees or "magnetic" degrees. If the distance is short, it shouldn't matter much.

[+Renegade Knight]

I'm doing a two-stage multi, where the second stage is a projection from the first. I find the first with no problem, but not the second. Others are finding it, so I know its there.

 

Here's my question: I'm using a Garmin 60 CSx. When it projects a waypoint, it does so using trig calculations, right? In other words, the unit's compass isn't used to project the way point, right?

 

Thanks

 

I can't answer your question, but I can tell you that you are not alone. I was trying to solve a cache the hard way and projected a waypoint. The point was projected to a point behind me and not where I was wanting to go. Once you know how to do it it's easy I supposed. Until then I need to see it on paper in clear terms so I know what the heck the GPS is asking me about.

[+imajeep]

I can tell you that you are not alone. I was trying to solve a cache the hard way and projected a waypoint. The point was projected to a point behind me and not where I was wanting to go. Once you know how to do it it's easy I supposed. Until then I need to see it on paper in clear terms so I know what the heck the GPS is asking me about.

 

I don't really have a problem projecting a waypoint. I use a Garmin 60CSx--to project a waypoint, hit the find button, select 'Geocaches', select the cache that represents the Stage One coordinates, then instead of hitting the 'Go To' button, hit the Menu button to show the pop-up menu. Select 'Project Waypoint', and a new waypoint will appear, with its distance field selected. Tab right to the 'mi' field and hit Enter to change units to feet. Then tab left to set the distance and bearing. It's really pretty easy.

 

What I've discovered this morning is that it makes a difference whether the GPSr is set to true north or magnetic north, even over a short distance. The cache I'm hunting didn't specify one or the other, so I've asked the CO which it is. Hopefully, that will put me in the right spot.

[+Sputnik 57]

Ok, I'm a dope. I admit it. If the declination in your area is high (say, 20 degrees) then you can get quite a difference, even over short distances. Two points projected 100 feet separated by 20 degrees would be about 35 feet apart. That's a pretty big search area.

[GeoBobC]

The Garmin 60 series can be set for either magnetic or true north. If you select magnetic, the unit will apply declination based on the units location. Go to Setup > Heading. I suspect this setting needs to match the projection bearing given to you. Another easy solution: project two waypoints, one for magnetic and one for true north. Find out which one is correct and use that method in the future.

[rhitchco]

I think you'll find it'll do much better if you wait until you get to the first waypoint and then project to the next one from there. Some of us were playing around with chained waypoints and there seems to be something like a compounding of errors going on.

 

If you put in all of the chained waypoints before starting, it will just get further and further off.

 

mogul2us

[+EScout]

I'm doing a two-stage multi, where the second stage is a projection from the first. I find the first with no problem, but not the second. Others are finding it, so I know its there.

Here's my question: I'm using a Garmin 60 CSx. When it projects a waypoint, it does so using trig calculations, right? In other words, the unit's compass isn't used to project the way point, right?

Thanks

 

Most GPSrs use the "Great Circle" calculations for projections (also known as Geodesic.)

 

As the others have said, you need to set your GPSr's north reference to magnetic or true as specified by the cache hider. Hider gave you this because it is an essential part of the Projection: Distance, Bearing (in True or Magnetic. Bearing might also be in degrees or mils.)

 

Great Circle Info

Edited July 25, 2007 by EScout

[+EScout]

Duplicate

Edited July 25, 2007 by EScout

[+imajeep]

Ok, I'm a dope. I admit it. If the declination in your area is high (say, 20 degrees) then you can get quite a difference, even over short distances. Two points projected 100 feet separated by 20 degrees would be about 35 feet apart. That's a pretty big search area.

 

I'm in Chicago--we have a pretty small declination. In this case, it came out to ten feet. But you're right, in other areas, with larger declinations, the distance could be larger.

[GeoBobC]

I live in the Pacific NW; declination is approximately 20 degrees (the angular difference between true north and magnetic north).

 

The difference between the two projections will be approximately the cosine of the angle times the distance. The cosine of 20 degrees is .94. If the projected distance is 100 feet, the difference between the two projected waypoints (magnetic and true north) should be approximately 6 feet. At 500 feet projection the difference would be approximately 30 feet.

 

Interesting note: magnetic declination is constantly changing, because magnetic north is moving. Where I live the shift is approximately 6 minutes per year or one degree every ten years.

[GeoBobC]

Oops. I double checked my math and found that I should have used the tangent rather than cosine. The difference between the two distances at 100 feet, given a 20 degree difference, would be about 37 feet. At 500 feet projection the difference would be approximately 181 feet. That's a lot of difference.

[+Sputnik 57]

I don't want this thead to go off on a "tangent," but that's the math I used too. Oh, and no, I won't "cosine" anything for you. :-)

[+Klemmer]

A bigger problem may be getting the projected bearing correct. I have seen it done 180 degress off at least once. It's a question of whether the cache owner did it FROM waypoint #1 forward TO waypoint #2, or if the owner projected FROM waypoint #2 back TO waypoint #1. Read cache page carefully, or query the owner if not clear. You might need to add or subtract 180 degrees to the given bearing. I ALMOST did some serious & not needed bushwhacking.

[+imajeep]

A bigger problem may be getting the projected bearing correct. I have seen it done 180 degress off at least once. It's a question of whether the cache owner did it FROM waypoint #1 forward TO waypoint #2, or if the owner projected FROM waypoint #2 back TO waypoint #1. Read cache page carefully, or query the owner if not clear. You might need to add or subtract 180 degrees to the given bearing. I ALMOST did some serious & not needed bushwhacking.

 

An excellent point. In this case, the CO projected back, but he subtracted 180° from the heading back to the original point in his posting instructions.

[+imajeep]

Oops. I double checked my math and found that I should have used the tangent rather than cosine. The difference between the two distances at 100 feet, given a 20 degree difference, would be about 37 feet. At 500 feet projection the difference would be approximately 181 feet. That's a lot of difference.

 

I'm glad I'm not the only person who does that. Our declination in Chicago is only 3°, changing by 5' per yr. The distance was only 143 feet, so you can see from the math that we're talking ten feet.

[+hogrod]

Our declination in Chicago is only 3°

 

I have a multicache that requires a bit of projection, 6months after I placed the cache I started to wondered why some GPSr would get the same numbers and others were off. This caches projection isn't over a great distance and the declination here in Wisconsin is only about 2degrees, yet the projection could be off 50ft or more depending on the TRUE/Magnetic north settings combined with unit signal quality/accuracy.

 

A little side note, I really like using GeoCalc for projections/waypoint conversions on the PC. It's such a simple program it can easily run it off a USB thumbdrive.