Dear Alex, Thank you for your rapid reply...
> As architect my comment on no.1. is that the north arrow which > we place in drawings (at least the ones I place) are based on > the North direction on large scale survey maps supplied by > planning authorities and on which the drawing would be based. That certainly makes sense. Here in the U.K. we would have to know whether the large scale map used true north or grid north but, subject to that caveat, your arrow ought to be within a degree or two of being right. The main reason for errors of 15 degrees or so seems to be a result of modern surveying practice using instruments which measure distance as well as angle... What seems to happen is that the surveyor takes ONE primary point on a building site and marks this on the ground with a pin and a little circle. This is the origin of an x,y,z coordinate system in which positive z is vertically up and the x-y plane is horizontal. This still lacks an orientation. In theory, positive y is due north and positive x is due east. Surveyors don't actually use the letters x, y, and z but talk about Easting, Northing and Height respectively. I have watched these guys at work. When they start off, they choose a reference origin quite carefully (you don't want a spot which is going to be built over or have huts on it) but they are much less fussy about orientation. They will simply guess which way is north (or east if that is more convenient) and slap a target on a wall and use that as their reference orientation for the whole construction period. If it is 15 degrees out no one seems to mind! Maybe surveying practice is more rigorous in Malta! Frank -
