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Understanding the Compass
Before subdividing a township, the surveyor was supposed to retrace the east line of Section 36 so he could correctly run the west line of Section 36.
If a surveyor retraced this line and found it to be N 0å¡ 4′ E and then stated that the variation was 13å¡ East, what was he specifically looking at on the dial to get these two numbers in relationship to True North?
Then, when he started out on the west line of Section 36 he set the angle on the compass at N 0å¡ 4′ E, ran the line and recorded the variation to be 13å¡ East. The first east-west line between section 36 and 25 was then stated to be Va. 13å¡05′ East.
Later, and further north between Sections 13 and 14, he wrote in his notes that he had evidently set off the angle as N 0å¡ 4′ W instead of N 0å¡ 4′ E. I’m trying to understand how an experienced surveyor could have this happen unless an inexperienced instrumentman got confused by the dial having NW to the right and NE to the left.
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41 Replies
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Wasn’t there a standard of practice to make an astronomic observation, at the outset of the township survey, and note the variation?
I have no idea. I had to look at a township map just to figure out where 36 was. -
What year and what PM are you in?
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The 13 degrees declination sounds reasonable for the midwest in the mid-1800’s. Much of Iowa was in the 10-11 degree range at the time. If he set the recorded variation (declination) and ran the mile of township line and his falling was off a little, I can understand how he could say the line was N 0å¡ 4′ E.
But I don’t follow the rest of the story. I’d have expected him to derive a new variation setting of 13+ degrees calculated to make him match the mile he just ran, and note that setting as he started subdividing the township. I’d have expected no further references to small values of variation.
His compass probably had a declination scale which I don’t see in your picture. It may have been called a vernier or nonius, but wasn’t used as a vernier on the sights like is done with a transit. It would have just been used to set the variation (declination) calculated to match the township line.
Did his compass have too little range on the declination scale, so he had pre-set the 13 degrees by altering the mount of the main compass dial? Then his small numbers would have been offsets to the 13. I haven’t heard of that, but it’s the only guess I can make sense of.
I hope someone can further enlighten us.
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The year is 1890 and the 5th PM in southern South Dakota.
I reread the beginning of the notes and the surveyor is using a Young’s Improved Solar Compass with Smith’s attachment. This seems a little surprising that he is using this instrument instead of a compass to break down the township. Perhaps it is his normal instrument that he uses. So, ignore the image I posted above of a regular compass dial.
Near the center of Section 11, before starting, he does an astro and states that the magnetic bearing of Polaris is N 3å¡10′ W. He then establishes a true meridian and the Variation is 13å¡28′ E.
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Where did the compass pictured above come from? I have never seen a compass with NW to the right of 0å¡. If I had to guess that is what a compass in the southern hemisphere looks like. I know Penry said to ignore the photo as it relates to the post but that compass intrigues me.
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McCracker, post: 354753, member: 9299 wrote: Where did the compass pictured above come from? I have never seen a compass with NW to the right of 0å¡. If I had to guess that is what a compass in the southern hemisphere looks like. I know Penry said to ignore the photo as it relates to the post but that compass intrigues me.
Look at any instrument mounted compass face from any surveying transit made in the 20th. century. There really is a good reason for the placement of the E & W…
this link may help:
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I understand that NE is also SW and NW is also SE in a heading. On a compass, I know there is a magnetic north and the declination between true and magnetic. From the link above , “Now let’s turn and face northeast. The compass needle doesn’t move, it always points north, and now it’s located half-way between the N and the E (northeast) on the compass. Let’s keep turning and face east. The compass needle is still pointing toward the north but now it lines up with the E on the compass indicating that we’re facing east. See why the E and W on compass faces are reversed?” I know that 90å¡ and 270å¡ are the same as far as a line and heading goes, however in the image above 25å¡ to the right of 0 equals 205å¡ and South (180å¡) is opposite 0. I can see where the “I-man” would have mis-read the direction as mine does frequently. Luckily modern CAD software has a translate rotate function. I want to understand, and would like to know where NW is to the right of N.
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J. Penry, post: 354738, member: 321 wrote: Near the center of Section 11, before starting, he does an astro and states that the magnetic bearing of Polaris is N 3å¡10′ W. He then establishes a true meridian and the Variation is 13å¡28′ E.
?? Transposition error ? The magnetic bearing of Polaris would be West of the pole (less the 10å¡ transposition error)
?? There was a mistake in whether Polaris was sighted at culmination or elongation ? The axis of the orbit wasn’t compensated for ? -
paden cash, post: 354756, member: 20 wrote: Look at any instrument mounted compass face from any surveying transit made in the 20th. century. There really is a good reason for the placement of the E & W…
I think you can go to most American made models since the eighteen century, and maybe earlier.
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So if “There really is a good reason for the placement of the E & W…” what is it? Not all forms of compass have this peculiarity. Is it only surveyors that need to have it? Do they often read the compass incorrectly because the information there is purposely confusing or obfuscating?
If you tell me to look at that compass then I can certainly see that the directions are reversed. That does not explain the reason why they are reversed. In some compasses the bezel of the compass dial is also reversed, showing azimuth marks from 0 to 360 in a counterclockwise direction rather than clockwise.
The fact that a compass was made this way since the eighteenth century also does not explain the need for this notation either. I have looked at several compasses and some are graduated to read the quadrant, some the azimuth, some in clockwise notation, some in counterclockwise notation, some with east and west reversed and some not. No wonder the surveyor or his assistant read the wrong bearing from the compass.
My favorite compass employs a mirror like the one in the compass transit tutorial referenced in an earlier post. Unless your compass is on a tripod or a staff the mirror can be used to level the compass, read the bearing or azimuth while sighting the object. It also reverses the reversed east and west directions on the compass.
I can read a compass. My question pertains to why it is necessary to have your own surveyor system of compass dial. Since other compasses such as those found in the binnacle of a boat or the compasses used by boy scouts and hikers do not purposely distort the cardinal directions wouldn’t this add to the general confusion?
Historic Boundaries and Conservation Efforts -
not my real name, post: 354798, member: 8199 wrote: So if “There really is a good reason for the placement of the E & W…” what is it? Not all forms of compass have this peculiarity. Is it only surveyors that need to have it? Do they often read the compass incorrectly because the information there is purposely confusing or obfuscating?
If you tell me to look at that compass then I can certainly see that the directions are reversed. That does not explain the reason why they are reversed. In some compasses the bezel of the compass dial is also reversed, showing azimuth marks from 0 to 360 in a counterclockwise direction rather than clockwise.
The fact that a compass was made this way since the eighteenth century also does not explain the need for this notation either. I have looked at several compasses and some are graduated to read the quadrant, some the azimuth, some in clockwise notation, some in counterclockwise notation, some with east and west reversed and some not. No wonder the surveyor or his assistant read the wrong bearing from the compass.
My favorite compass employs a mirror like the one in the compass transit tutorial referenced in an earlier post. Unless your compass is on a tripod or a staff the mirror can be used to level the compass, read the bearing or azimuth while sighting the object. It also reverses the reversed east and west directions on the compass.
I can read a compass. My question pertains to why it is necessary to have your own surveyor system of compass dial. Since other compasses such as those found in the binnacle of a boat or the compasses used by boy scouts and hikers do not purposely distort the cardinal directions wouldn’t this add to the general confusion?
Gawd, you kids are going to drive me crazy with this…..
First, it is NOT really a compass, it is a surveying instrument. The instrument is what moves, NOT the needle. The E & W are placed on the face of the needle circle so the instrument operator can read the bearing DIRECT. If you turn the instrument to the NW, it will read a NORTHWEST BEARING. If it were a compass the needle would indicate an erroneous NE bearing.
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not my real name, post: 354798, member: 8199 wrote: So if “There really is a good reason for the placement of the E & W…” what is it? Not all forms of compass have this peculiarity. Is it only surveyors that need to have it? Do they often read the compass incorrectly because the information there is purposely confusing or obfuscating?
If you tell me to look at that compass then I can certainly see that the directions are reversed. That does not explain the reason why they are reversed. In some compasses the bezel of the compass dial is also reversed, showing azimuth marks from 0 to 360 in a counterclockwise direction rather than clockwise.
The fact that a compass was made this way since the eighteenth century also does not explain the need for this notation either. I have looked at several compasses and some are graduated to read the quadrant, some the azimuth, some in clockwise notation, some in counterclockwise notation, some with east and west reversed and some not. No wonder the surveyor or his assistant read the wrong bearing from the compass.
My favorite compass employs a mirror like the one in the compass transit tutorial referenced in an earlier post. Unless your compass is on a tripod or a staff the mirror can be used to level the compass, read the bearing or azimuth while sighting the object. It also reverses the reversed east and west directions on the compass.
I can read a compass. My question pertains to why it is necessary to have your own surveyor system of compass dial. Since other compasses such as those found in the binnacle of a boat or the compasses used by boy scouts and hikers do not purposely distort the cardinal directions wouldn’t this add to the general confusion?
From the link of the tutorial: “Let’s take a moment to make sure we understand how such a compass works. Do you know why, for example, the E (east) and W (west) appear reversed on the compass above?”
If you indeed read this and observed (while thinking) the diagrams below the statement, I cannot understand your confusion.
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J. Penry, post: 354738, member: 321 wrote: The year is 1890 and the 5th PM in southern South Dakota.
I reread the beginning of the notes and the surveyor is using a Young’s Improved Solar Compass with Smith’s attachment. This seems a little surprising that he is using this instrument instead of a compass to break down the township. Perhaps it is his normal instrument that he uses. So, ignore the image I posted above of a regular compass dial.
Near the center of Section 11, before starting, he does an astro and states that the magnetic bearing of Polaris is N 3å¡10′ W. He then establishes a true meridian and the Variation is 13å¡28′ E.
Don’t you suppose that the solar compass was used to get the variation, but that the lines were actually run by the needle? If so, any error in the variation needed to retrace a particular line would almost certainly be calculated from the fallings, i.e. by running a trial line at a certain variation and from the miss at the end of the line computing the variation at which the line should have been run to hit the corner. That is how that problem was worked in Texas in 1890.
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Kent McMillan, post: 354807, member: 3 wrote: Don’t you suppose that the solar compass was used to get the variation, but that the lines were actually run by the needle? If so, any error in the variation needed to retrace a particular line would almost certainly be calculated from the fallings, i.e. by running a trial line at a certain variation and from the miss at the end of the line computing the variation at which the line should have been run to hit the corner. That is how that problem was worked in Texas in 1890.
OMG Kent…have you been studying to take the Public Domain exam in some PLSS state???
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paden cash, post: 354801, member: 20 wrote: Gawd, you kids are going to drive me crazy with this…..
First, it is NOT really a compass, it is a surveying instrument. The instrument is what moves, NOT the needle. The E & W are placed on the face of the needle circle so the instrument operator can read the bearing DIRECT. If you turn the instrument to the NW, it will read a NORTHWEST BEARING. If it were a compass the needle would indicate an erroneous NE bearing.
It is a compass. Like any compass the needle does not move. The needle points to magnetic north. Do you understand what you are saying? All compasses have needles that do not move.
Surveyors invented some kludge way of calibrating a compass so that it reads “direct” and is a source of confusion for everybody else. Like I said before… I can read a compass. It is not necessary to kludge up the dial in order to read a bearing on any compass. Especially with a compass that has a needle which does not move.
Historic Boundaries and Conservation Efforts -
The compass (or whatever you call it) from the original post above appears to be showing a bearing of 24 and one-half degrees northwest on the line of sight. It sure looks like a compass to me, but, I am not from 1880 like you, but I can still read a compass. I do think the dial is easily confusing and not necessary.
Historic Boundaries and Conservation Efforts -
not my real name, post: 354825, member: 8199 wrote: The compass (or whatever you call it) from the original post above appears to be showing a bearing of 24 and one-half degrees northwest on the line of sight. It sure looks like a compass to me, but, I am not from 1880 like you.
Historic Boundaries and Conservation Efforts -
not my real name, post: 354825, member: 8199 wrote: … It sure looks like a compass to me, but, I am not from 1880 like you, but I can still read a compass. I do think the dial is easily confusing and not necessary.
I’m really trying to be helpful, but Boy Scouts can read compasses. apparently you’ve never stood behind a surveying instrument. If you had, you’re remark about the layout being “confusing and not necessary” would seem almost ignorant (I am not calling you ignorant).
I sure hope you’re not actually in some sort of supervisory capacity of any field work concerning survey retracement.
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One can build a better mouse trap but it will most certainly still confuse some mouseketeers.
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