Finding RMs Via "Geodetic Azimuth" ?

[+pgrig]

I'm not sure I understand what looks like an important concept.

 

Yesterday I was searching for MY2614 and its three RMs.

 

In order to locate RM-3 from the station, it seemed that I needed to add the declination back into the "geodetic azimuth" given in the description then take a back bearing (the given bearing minus 180 deg.) to get the compass bearing from the station. Using this technique, I found the RM-3 bolt, but I didn't succeed in finding RMs 1 and 2. Of course, the number of mosquitoes feasting on me while I worked might have had something to do with it...

 

Help, please.

-Paul

[Bill93]

geodetic azimuths are essentially true to the earth's axis, and you need to adjust a compass by declination to get it to match.

 

Modern azimuths are from true north. An older convention gave geodetic azimuths from south. The azimuths on a data sheet are now from true north (see the box score of the sheets off the NGS site), but older recovery reports will sometimes have azimuths from south in their text.

 

For MY2614 the box score gives the azimuth to the church as about 340 degrees (NNW) whereas the 1935 notes say 160 degrees. This proves the necessity of reversing all the other azimuths in the notes.

Edited July 30, 2008 by Bill93

[+jwahl]

It sounds like you have one solution. If you do not have a compass that allows you to set off the declination so that IT reads true azimuth, then a good alternative is to convert the true bearings to magnetic.

 

Let me give an example off the top of my head. If the declination is west 11 degrees, that means that the needle on the compass points to the west of true north and with the compass pointed true north the compass needle would read 349 degrees. Stated another way when your compass is pointing magnetic north or Az of 360 Geodetic Azimuth is 349 degrees. 360m = 349g.

 

So for a West declination take the true azimuth and add the declination value 349 + 11 = 360 magnetic.

 

For an East declination you would subtract its value.

 

Or if you apply a sign convention of W is - and E is plus you would always subtract 349 - (-11) = 360 to go from true to magnetic. Of course the opposite applies going from a magnetic reading to true.

 

Use any of a number of links available to obtain the current declination for your approximate location and current date.

 

In MA I would expect a westerly declination of about 15 degrees 10 minutes or 15.2 degrees.

 

Another possible way to do this is the conversion can be done in many GPS units. Garmins will show you the declination from their built in mag model for wherever you are and can also just be set to show magnetic bearings.

 

Your RM 1 is listed as azimuth 209-47 (rounded) geodetic azimuth. Following my rule I would add 15d 10' to get 224-57 magnetic, in practice rounded to 225 degrees.

 

- jlw

 

PS: It is pretty easy to get which direction to apply the difference confused. Each type of compass has a preferred way of using it, but a compass that can set off the declination is desirable.

 

I'm not sure I understand what looks like an important concept.

 

Yesterday I was searching for MY2614 and its three RMs.

 

In order to locate RM-3 from the station, it seemed that I needed to add the declination back into the "geodetic azimuth" given in the description then take a back bearing (the given bearing minus 180 deg.) to get the compass bearing from the station. Using this technique, I found the RM-3 bolt, but I didn't succeed in finding RMs 1 and 2. Of course, the number of mosquitoes feasting on me while I worked might have had something to do with it...

 

Help, please.

-Paul

Edited July 30, 2008 by jwahl

[Papa-Bear-NYC]

geodetic azimuths are essentially true to the earth's axis, and you need to adjust a compass by declination to get it to match.

 

Modern azimuths are from true north. An older convention gave geodetic azimuths from south. The azimuths on a data sheet are now from true north (see the box score of the sheets off the NGS site), but older recovery reports will sometimes have azimuths from south in their text.

 

For MY2614 the box score gives the azimuth to the church as about 340 degrees (NNW) whereas the 1935 notes say 160 degrees. This proves the necessity of reversing all the other azimuths in the notes.

True - most old reports give what we would now call reverse azimuths (azimuth from South is another way of saying it).

 

Another convention often used is azimuth east or west from north or south. Let me explain. For a true azimuth of 20 degrees, they would say North 20 degrees East. For a true azimuth of 160 degrees they would say South 20 degrees East. For true azimuth of 340 degrees they would say North 20 degrees West, etc. It's not hard once you get the pattern, but you have to be VERY careful, or you'll be way off. I believe this way of specifying directions is still widely used. It's probably a result of the instruments they used. They would set the transit (or theodolite) to either north or south, which ever is closer, and then rotate it east or west to sight the target.

 

The most confusing cases are when the use that method of specifying the azimuth AND they use reverse azimuth. So the statement RM1 is 26.34 meters South 20 degrees East from the station. And the true azimuth is 340 degrees! Figure that one out! (hint: for reverse azimuths substitute North for South and West for East. Simple )

 

Luckily the box score is consistent and easy to understand. (although I've seen mistakes in a box score or two).

Edited July 31, 2008 by Papa-Bear-NYC

[+pgrig]

Thank you all!

 

So bottom line, it looks like I did do the right thing in the field . It just goes to prove that even a blind squirrel finds a nut once in a while...

 

In this case, I was not looking for the Primary Azimuth Mark (since, as in this case, this mark is often obscured for the stations I hunt, since they are generally 25-50 years old).

 

I don't believe I often see descriptions with so many RMs called out by azimuth, and I guess not by "geodetic azimuth". In the future I will be more careful about remembering to RTFD (read the description) in advance. That seems to be the solution to many problems.

 

I use a Silva Ranger compass with a declination offset screw (left over from my Scoutmaster days), but I wouldn't like to tinker with the setting in the middle of the woods, thus forcing me to do mental math games (which are not my forte).

 

So I guess the term "geodetic azimuth" is equivalent to the term "true (non-magnetic) azimuth" (or bearing), leaving the question of whether it's a bearing from North or South. Glad that was simple! By the way, can someone tell me when to use the term "bearing" vs. "azimuth"?

Edited July 30, 2008 by pgrig

[+Black Dog Trackers]

By the way, can someone tell me when to use the term "bearing" vs. "azimuth"?

I think they are pretty much interchangeable. Bearing has a bit more of a meaning of going or planning to go in a particular direction, whereas azimuth is more used for looking in a particular direction.

[tosborn]

An azimuth is an angle measured from a meridian (usually North) going clockwise from zero to 360 degrees through a full circle.

 

A bearing is an angle measured within a given quadrant (e.g. NE, NW, SE, SW). In the NE and NW quadrants the angle is measured from North toward East or West. In the SE and SW quadrants the angle is measured from South toward East or West. A bearing will never exceed 90 degrees. Bearings are typically used in property descriptions. It's easy to express a back-bearing....you just reverse the quadrant. For example the back bearing of N35E is S35W.

[+Black Dog Trackers]

Well on Star Trek, Spock announced tricorder bearings for any angle, not just up to 90.

[+pgrig]

Then there's Run Silent, Run Deep: "Range 4000 feet, bearing 270 degrees, angle on the bow 14 degrees. Ready tubes 1, 2, and 3. Fire!"

[+GEO*Trailblazer 1]

Double

Edited July 31, 2008 by GEO*Trailblazer 1

[+GEO*Trailblazer 1]

That is why I use GPSTrackmaker.

 

It has an Azimuth tool which you can use to get a precise bearing to target.

It takes a little playing to get use to but I have all the data before I go out even a calculated waypont for all marks.

I use a multi combo and the data sheet how to get to to determine how far usually.

You can also back sight before ever leaving home.

 

I just can't get out as much.

[+Klemmer]

Bearings can be relative or true or magnetic (depending on the industry / application). Relative bearings are often used in the aviation and marine business. In those cases, they are normally relative to the nose / bow of the aircraft / vessel. E.g.: "your traffic is bearing 045 degrees at 30 miles". That's to your forward right (starboard).

 

True & magnetic pretty well covered above.

 

Azimuth is sort of a special case of a bearing. It's a true bearing from the station (fixed location).

[+PFF]

For finding reference marks, don't overlook a valuable tool called FORWARD, available from the "Tool Kit" on the NGS website. (It can be used on-line, or downloaded.)

 

It will calculate the latitude and longitude of a reference mark so precisely that you can walk right up to it. Everything you need is in the "box score" for most triangulation stations.

 

The exception is the Az Mark, which seldom has the exact distance. I use the approximate distance, as given in the remarks, and I plot several points to create a line. For instance, if the distance is 0.35 mile from the station, I create waypoints at 0.3 mile (482.8032 meters) and 0.4 mile (643.7376 meters). Sight along the line, looking for objects mentioned in the description. HINT: Since Az Marks often are along a roadway, take note of where the line crosses a road. Then see what the remarks say about how far (and in what direction) the mark is from the centerline.

 

I realize this method is not used by some folks because it removes the challenge of the hunt. And I admit to sighting with the compass and pacing off the distance, at times. But with the computer doing the work, I have the reference mark coordinates pre-loaded most of the time. With NGS FORWARD, the phrase "As easy as shooting fish in a barrel" comes to mind. And if there are obstacles between the station and its reference marks, this may be the only way to find them. (See story, below.)

 

-Paul-

On a recent day trip, one of the marks I'd be hunting was a triangulation station set 50 years ago. There were no recovery notes for over 40 years. Aerial photos revealed that the once remote mountain top was now develped. It appeared that if I could park at the last house on the road and walk across the yard, I'd be very close. I made note of the owner's name before leaving home.

 

That afternoon, I knocked on the door and asked permission to leave my car in his driveway for a few minutes. When I explained what I was doing, he said he knew where the disk was, and he offered to show me. As we walked into the woods, he said, "You know, there are TWO of those disks up here." I mentioned that originally there had been three. "I've heard that, too,", he responded, "but I've never found a third one." I answered that perhaps today would be the lucky day.

 

The first mark we came to was a reference mark, and then after hopping over some downed trees, we came to the station. After taking photographs, I said I'd like to look for the third disk. Calling up the waypoint in my GPSr, we set off in the general direction. We had to detour around debris from past ice storms and hurricanes. But I kept following the needle. I could see we were getting closer, and as I stepped over one more tree, my foot came down on the monument.

 

"Wow! You're good!" my host exclaimed. I grinned and responded, "Remember....I'm very experienced. Don't try this yourself at home!"

[+seventhings]

Occasionally, I will use the method described by PFF, above, to find a reference mark. It is especially helpful if the RM is more than 50 feet (about 15 meters) or so from the station. I find that it's not very helpful if the RM is within 35 feet (10 meters) of the station and/or I'm working under tree cover or otherwise have a poor signal. With a poor signal, the CEPs of the station and the RM overlap if they're within 35 feet.

 

Another technique I use is this: From the boxscore on the NGS datasheet, I note the azimuth and distance (converted from meters to feet) for each of a station's RMs. For example, let's say t-sta BITSKO has an RM at an azimuth of 245 degrees [true from the station] at 23 meters (75.5 feet). Then, I find the station (sometimes with or usually without help from the GPS). With the station's adjusted coordinates in the handheld and the handheld set to GOTO the station, I start at the station and walk generally a bit south of due west.

 

I'll alter my course northerly or southerly to get the "tail" of the GOTO arrow to move toward and settle on 245 degrees. [Remember, if you travel in a straight line, the head of a GOTO arrow always "falls" toward the bottom of the compass rose and the tail always rises toward the top. This is an old, round-dial radio beacon navigation technique with which old, round pilots are familiar.] So, for example, if the tail of the GOTO arrow settles on 235 degrees, I'll walk more northerly until the tail rises to 245 degrees. Finally, I walk "outbound" on the 245 degree azimuth (radial in pilot talk) until my GOTO says I'm 75.5 feet from the station. Nine times out of ten, I'll be within six feet of the RM.

 

Were I out on the prairie (really good signal) and the RM was 120 feet from the station, I would more likely use the NGS FORWARD program to calculate the RM's coordinates. From my experience, FORWARD calculated coordinates are alomst as accurate as adjusted coordinates (at least as sensed by my handheld which is plus or minus 10 feet under most good conditions.)

 

Will

Edited August 4, 2008 by seventhings

[+Klemmer]

Hey! I resemble that remark about old round pilots! I've homed on a goodly number of various kinds of beacons. Even landed a few.... I agree completely with your terminology & techniques.