PolarAlign

It's easy to establish an accurate North-South line (meridian) during the day. 

The premise is that at the time of the solar transit (highest place in the sky), the sun is absolutely due south. 

Basically you use a weight and a string hanging from a tripod, then trace the shadow made by the string at the moment of solar transit.

This method is taken largely from Brayebrook observatory.  The manner described in that site is math intensive. At the time it was written, calculations were necessary to find transit times for specific coordinates.

Now there are  websites that will give you the times directly so calculations are no longer necessary.

Tools

1. Plumb bob
2. Tripod
3. Large piece of paper
4. Felt tip marker
5. Weights for the paper
6. Accurate watch
7. Straight edge
8. String

 Information you need

1. Accurate geographic coordinates
2. Your altitude
   
3. Your offset for GMT
   
4. Time of transit

It's easiest to use a GPS to get accurate coordinates for your observatory. Your GPS may give you your altitude as well.  If not, just get close.

You need the time zone offset from GMT.  For U.S.A. Eastern Standard Time, it is -5 hours. If you need your offset, try this site.

You need time of transit.  This website will give it to you accurate to the second.  Most other sites are accurate to only to the minute.   This site is a little confusing because some times are local, some are GMT.  The transit time is GMT, so take that time and correct for your offset.  The time given to me today is 17:53:27.  Since I'm using EST, I'll subtract 5 hours to give 12:53:27. 

Here's another site that will calculate transit time, you can use this one to double check the first one. On this second site, make sure you hit <enter> after you input each of your coordinates or it won't update the screen and the times will be wrong.

Astronomy software will likely give you solar transit times. Usually you select the sun and click the option that gives you more information about the selected object.   Look for "transit".

With this information, you're ready to start.

First establish where your mount will be.  You want the point exactly underneath the two axis of rotation of the mount.  Mark this spot on the ground.

Set up the tripod over the spot. Keep in mind the shadows of the legs of the tripod, you don't want a tripod leg shadow overlying your string shadow.

Hang the plumb bob so the point is over the marked spot on the ground.

Place a large piece of paper under the plumb bob.  The paper should just extend just a few inches south of the tip of the bob and extend to the north.  The shadow will extend from the tip of the bob northward so you want the paper mostly to the north.

Weight the paper in position.

Mark a dot under the point of the bob.

Draw a line horizontally near the north end of the paper. This is to keep your marks on a straight line across the paper in case you need to interpolate between two dots. Interpolation will only be necessary if there's a problem at the second of transit (cloud, double shadow, missed the time, etc.).

Starting 1/2 to 1 hour before transit, mark the point where the shadow crosses your line at regular intervals such as 12 minutes.  I've used 1 minute intervals for the first 6 minutes then 6 minute intervals from then on.  Since my transit time shadow was good, I stopped with the transit time measurement. Mark the point at the time of transit within a second or two.

If clouds obscure the shadow at transit time, continue to mark times for 1 or two periods after transit then interpolate the transit point.

Draw a line from the point under the bob to the point you marked at the transit time.

Extend a string along that line in both directions.  This line is true North-South.  Mark the line accurately and permanently.

Your mount can now be installed at any point along the line.

Answers to questions:

Daylight savings time alters the GMT offset by an hour in a positive direction.  If you are in the Eastern US time zone, the standard offset is -5 but if you are using daylight time, your offset would be -4 instead.

For latitude, northern hemisphere is positive, southern hemisphere is negative.

For longitude, those values listed as "W" are negative and those listed as "E" are positive.  For example, you could list the longitude of  Atlanta, Ga, USA as either W84.42 or -84.42.

The site has been marked and the plumb bob hangs over the mark.

The paper is under the bob, and the paper marked with a dot.

It can be difficult to precisely place a dot under the bob, here a washer is used and centered under the bob. The washer is held in place, the bob moved, and the dot marked.

Uh oh, a double shadow

If the sun is behind a tree, the sun is divided into 2 parts by the tree, each part of the sun casts its own shadow

Timed marks are made on the paper. Each tiny dot is one minute. It is surprising how quickly the shadow travels.

The time associated with the solar transit is marked and a line drawn from that mark to the mark under the bob. This line is true North-South. If a cloud obscured the sun at the time of transit or if a double shadow occurred at the time of transit, measurements would have continued and the transit time interpolated between two accurate single shadow dots.

Next the line is extended past the paper and a mark made on the concrete on either side of the paper.  The paper can then be removed.  The two marks are on the true N-S line.  Next I snapped a chalk line over the two marks to recreate the line on the concrete.