Surveyors

[LewisM]

A serious question - anyone know a rough figure to have a surveyor come to a house and in certain locations exactly map and mark true south?

The compass I use - a Silva - whilst OK, is far from accurate enough - user guesstimate more than anything. Tried a GPS, but again, not really precise as again, user error.

Was thinking if got a surveyor out with a laser theodolite etc...

Solar noon puts me within 2 minutes az on average. I would rather that was 2 secs :)

[Kunama]

Cost me 2 cartons of XXXX when I lived in Mooloolaba.

Lewis, go to the nearest construction site and ask the surveyors there. Most of the guys we had would do something like that on their way home for beers.

[tlgerdes]

Why is the big question?

Drift align one night and mark it yourself in the ground. Mark the ground for your tripod and take note of exact measurements for the tripod and mount setup including altitude and azimuth settings.

Remember as soon as you pack up, your new setup will most probably still be around 1-2 min arc acurate when putting it back down on the ground. Especially if you have used it in an alternate location.

[LewisM]

Already do that Trevor,I just want a really accurate N-S line.

Like I said, I am close as is, but... :)

[rmuhlack]

if you are setting up and packing up each time, is it even possible to achieve an accuracy better than 1-2 arcmins (even with markers on the ground) ??

A reality check from my back-of-the-envelope calcs - to achieve an alignment error of ~2arcmins, the tolerance for placement of my EQ6 tripod on the ground is about 0.22mm with the legs fully retracted. Based on that I'd have my doubts about whether I could repeatably eyeball the mount placement any more accurately than that, even with markers on the ground. :shrug:

[Stardrifter_WA]

I am with you Richard, I think it is near impossible to get complete accuracy by lining up tripod legs to marks in the ground by sight. I have given this problem some thought, as I am in the same situation. The solution I am going to try, although not perfect, is to line up, as accurately as I can, the SCP by drift method, mark where the legs are and then I am going to drill holes in the concrete to take a large (3") bolt, that will sit flush with the concrete and that will be chemically achored. The head of the bolt will have a steep cone recess machined into it (similar to a "centre drill" cone), in order to positively situate the point of the tripod legs each time in the same position. Once that is done, I will situate each leg in these cones and once again drift align using the fine azimuth adjustments. In this way, I should, hopefully, be able to set it up fairly accurately each time. Because of possible vibration problems with this idea, I may substitute bolts with rubber or a rubber/bolt combination, but I have to give this some more thought.

Although I have a permanent set-up in my dome, I need this so that I can quickly set up my refractor next to my dome, without the need to set up a permanent pier, which would be ideal, but in fact would be in the way. So, the above is the only solution I can think of, which isn't too bad an idea, as I can have the same arrangement out the front of my house as well, for those objects I just cant see from my back yard. Also, if this works, I may possibly be able to have the same arrangement at my dark sky site.

Cheers Peter

[kinetic]

Here's how I zero out any misalignment in my albeit permanent setup.

http://www.iceinspace.com.au/forum/showpost.php?p=637172&postcount=7

This method can be applied to a semi permanent set of marks / pavers
on a lawn. As others have said, unless you are going very narrow field
it just aint worth worrying to much about precision.

[Wavytone]

Lewis,

Simple surveyors trick: think of the scope as a theodolite. You could use the telescope itself but the magnification is far too great - 6-9X as per the average finder is perfect for this.

Years ago when i lived with a big garden, having got the mount aligned I set the mount so the dec axis was precisely horizontal (using a bubble level) then swung the OTA down (ie rotate the dec axis) till I could see the top of the fence in my finderscope*.

Waving handsignals at my devoted chain-man (wife) who was at the fence, she was able to put a mapping pin (a nail will do) at the spot indicated by the finders crosshairs.

Next time you set up the mount, to set the azimuth accurately look through the finder and rotate the mount in azimuth until the nail is on the finder crosshairs.

This assumes the mount is in the same spot each time, and the finder crosshairs and telescope have been accurately aligned.

BTW I worked a chainman in my student days, fun job...

[rmuhlack]

Further to my earlier post, to achieve an AZ error of ~2 arcsecs would translate to a tripod placement tolerance of < 4 microns.

ie practically impossible to do repeatedly

[LewisM]

My set up is a triple checked solar noon line, and tripod leg markers (I don't fold the tripod, so they are always at the same distance, same levelling setting etc).

I use the tripod in reverse so to speak - I have the traditional polar leg at the REAR (never repositioned the az peg), and then I have an aluminium tube in the polar alignment scope opening, from which hangs a plumb bob on a string. I then line up the leg and the bob on the solar noon line. I also check it with a compass - the compass is permanently affixed to a 2 metre long aluminium flat strip, bent at the rear to exactly my mag var'n, and fitted with a dovetail so that it fits the mount. I can then recheck to see if it is close (I got the alum bar idea from a post here some years ago - I think it was Lars?)

Been rewarded with near spot on alt, and max 2 min off in az. Maybe good, but I really would like it better, as yes, I DO want to go deeper yet.

Nothing wrong IMHO with wanting accuracy. Near enough is not always good enough for everyone

[Wavytone]

Your telescope and its mount will bend more than that - starting with the mirror support (newtonians)m the diagonal spider, where the dovetail is attached to the OTA (a real weak point on most scopes), the dovetail itself will twist more than that...

... even the tripod legs will flex that much in a breeze.

[Wavytone]

I tried that initially to get close - it is crude and inaccurate.

[kinetic]

<facepalm>

[Varangian]

Have you considered differential DGPS? I have one and contacted the manufacturers of my YUMA unit here at Ultimate Positioning in Melbourne today to assess your question and they said that once the unit has acquired the necessary data you will get true south as close as you ever will any other way. If you were in Melbourne I would come and do it myself, maybe look into half day hire? All you need to do is turn it on, acquire the spatial data and use the compass. Some units (mine included) project a laser once data is acquired.

Just a thought.

[LewisM]

Newtonians? Me... nah, only refractors here - a medium length OTA and a short little bugger. Not much flexing going on :)

[LewisM]

Helpful :)

[LewisM]

OK, OK, save everyone gettin' all in a tizzy, I'll just keep my ideas to myself :)

[tlgerdes]

Now, now, Lewis don't be like that. We are all here to share ideas. Some of these things we have all been through before, and have led us down rabbit holes that dont end.

Requirements-Conceptual Plan-Logical Design-Physical Design :question:

We are just offering different solutions to your requirements. Polar alignment better than 2arc/min.

We are just saying unless you have a permanent mount you are probably not goign to get much better without an accurate setup plan.

Just trying to save you some pain, but if you have your heart set, go forth and survey. :thumbsup:

[LewisM]

Well, I WAS considering a removable pier in the front garden - at least the tree will block the darned streetlight there!

Tried doing it from my ULTRA dark hidden courtyard... problem was the alignment - so few were able to be seen :LOL: - had to run 2 star alignments only, using Sirius and Achernar - not the best triangulation for sure!

[Wavytone]

Lewis, DGPS is one way. Without DGPS there are other ways:

(1) From your longitude, calculate when local solar noon occurs, to 1 second. Instead of using a shadow, set up a small lens with a long focal length to cast an image of the sun on card as it passes through local solar noon. Assuming the lens is fixed, project the image if the sun onto a filing card that has a circle drawn on it the size of the sun, and a vertical line (north-south) through the centre.

Manually move the card to follow the suns image keeping its disk centred on the circle. At local solar noon, stop moving the card. The central line gives you a very accurate measurement of where the meridian is.

Reading glasses of 0.5 dioptre from a chemist will do nicely, put a diaphragm in front to stop the lens down to a 1 cm diameter. For a focal length of 2 metres the image will be about 20mm across. With care you can make the measurement with an accuracy about 1 mm - which corresponds to about 1-2 minutes of arc since the sun is about 30 minutes across - far, far better than using shadows.

(2) If you can obtain 1:50,000 or 20,000 topographic map of your locality, or even better, the surveyors "deposited plan" or DP of your block of land - plot your telescope position on the map and find a distant landmark on the map visible from your telescope position, and put a nail (1) where this intersects your fence. Determine the azimuth accurately using the map coordinates. Using trigonometry, calculate exactly where true north or south intersects your fence.

Google maps or the stuff you get on smartphone apps are not sufficient for this.

Using your finder scope to get a bearing on the distant target, use a tape measure to measure the offset from nail (1) along the fence and put a nail (2) there. If you are using a fork mount, this should give you true north/south fairly accurately. If using an equatorial, you will need to make a correction for the lateral displacement of your finder from the RA axis.

If you have the DP for your block of land (this is usually included in the contract when it was purchased) take a close look at this. There are often measurements and precise bearings shown from boundary pegs to surveyors marks nearby, these can be crows-feet or drill-holes in the kerb or to trig stations if visible from your block. The worst case is when they reference a survey pipe buried under a pavement in which case you are out of luck.

(3) Another method if you don't have an accurate map, but (a) you do have a GPS (smartphones or iPads will do) and (b) you can see a distant landmark. Using GPS measure the lat and long of your telescope. Then measure the position of the landmark (go there). Note that the smartphones and commercial GPS accuracy will be about 3-5 metres at best, so to get accuracy around 1 minute of arc the landmark must at least 2km away. Using trigonometry, calculate the bearing from your mount to the landmark. Then work out where north/south is with respect to the land mark as measured along a fence, and proceed to use nails in the fence as in method 2 above.

[LewisM]

Thanks - what I was looking for, and no face palms :)

I'll look into DGPS too.

[pgc hunter]

For a surveyor to find actual true north, it is quite an involved process. Ideally, they have to find 2 permanent marks in the near vicinity with a known bearing between them based on the GDA94, and assuming this is the case, the surveyor would then place a temporary benchmark (TBM) on the desired location on your property.

Then they would set their total station up on top of the permanent mark with known coordinates, and sight to the other mark (by using a prism mounted on a tripod on the other mark) and then input the GDA94 bearing into the machine. Then the procedure is to traverse from the original station to where the scope will be placed. Common procedure for this is once a bearing is established, a second prism will be placed on a temporary benchmark located towards the intended target, that is a point selected by the surveyor. With a known GDA94 bearing now input into the total station, they will have to traverse to the desired location. Prisms will be placed either side of the total station - toward the intended target and back "behind" it on the previous mark, this can be either known permanent marks or temporary marks that the surveyor creates. Using the original bearing, the surveyor will swing the total station to both next and preceeding mark, both clockwise and counterclockwise (foresight and backsight), for a total of 4 readings between each mark, known as "face left" and "face right". This is done for accuracy and redundancy. The total station, using the original bearings between the two known points, will show accurate bearings to each successive mark. Eventually, the total station will end up on the final temporary mark (which will mark the location of the telescope) , and by simply sweeping horizontally with the station using the bearings already found, the station will display the true bearing as it is swung around. True north can now be found to an accuracy of less than 5 arc seconds.

An alternate method is using northing and easting coordinates. Bearing and distance can be found using the eastings and northings of two points. This is handy if there are no known bearings between two marks, It is possible to work out the required parameters using mathematical formulas. Then the surveyor can proceed to find any other point they desire as per the process I described above.

[LewisM]

Thanks Sab. I contacted the local uni engineering dept. to see if they wanted a field exercise for the engineering/surveying students :)