Level (optical instrument): Difference between revisions

The surveyor looks through the eyepiece of the telescope while an assistant holds a vertical [[level staff]] which is a graduated in inches or centimeters. The level staff is placed with its foot on the point for which the level measurement is required. The telescope is rotated and focused until the level staff is plainly visible in the crosshairs. In the case of a tilting level, the fine level adjustment is made by an altitude screw, using a high accuracy bubble level fixed to the telescope. This can be viewed by a mirror whilst adjusting, or the ends of the bubble in a "split bubble" display can be viewed within the telescope. This also allows assurance of the accurate level of the telescope whilst the sight is being taken. However, in the case of an automatic level, altitude adjustment is done automatically by a suspended prism due to gravity, as long as the coarse levelling of the instrument base is accurate within certain limits.

When level, the staff graduation readingreadings at the crosshairs and stadia marks are recorded, and an identifying mark or marker placed where the level staff rested on the object or position being surveyed.

In 1832, [[English people|English]] [[civil engineer]] and inventor [[William Gravatt]], who was commissioned to examine a scheme for the [[South Eastern Railway, UK|South Eastern Railway's]] route from London to [[Dover]], became frustrated with the slow and cumbersome operation of the "Y" level during the survey work, and devised the more transportable, easier -to -use "dumpy" level,<ref>{{cite book|title=Surveying|author=Saikia|publisher=PHI Learning|date=30 Oct 2010|isbn=8120339851978-8120339859|display-authors=etal}}</ref><ref>{{cite web|url=http://www.craig-telescope.co.uk/life_gravatt.html|title=William Gravatt|publisher=craig-telescope.co.uk|accessdate=2012-05-31}}</ref> so called because of its shorter appearance.<ref>{{cite book |title=Oxford English Dictionary |location="dumpy", adj, 2 |quote=Gravatt's Improved Level, commonly called (from its appearance) the Dumpy Level.}}</ref>

The telescope of the historic "yY" level is held in two brass arms, which are part of the mount and the telescope could be easily removed to allow sighting reversal though 180 degrees or an [[axial rotation]] of the telescope; both to compensate for optical [[collimation]] errors. Because the telescope is not fixed to the level adjusting mechanism, the "Y" instrument is assembled and disassembled for each sighting station. However, the dumpy level is permanently secured to its two support arms and the levelling mechanism, thereby reducing measurement uncertainty and considerably reducing the time taken to set up the instrument. The Dumpydumpy uses the same basic principle of level sighting.

Sighting is done with an assistant surveyor who holds a graduated [[Level staff|staff]] vertical at the point under measurement. The surveyor rotates the telescope until the graduated staff is in the cross hairscrosshairs and records the reading. This is repeated for all sightings from that datum. Should the instrument be moved to another position within sighting distance, it is re-levelled, and a sighting taken of a known level in the previous survey. This relates any new levels to the previous levels. The backsight level reading is always subtracted from the foresight level reading, and that equals the change in elevation between the two points, which is why this method is referred to as '''differential levelling'''.

The term '''dumpy level''' (also '''builder's level''') endures despite the evolution in design. They can be manual or automatic -, the latter being much quicker to set up.

A '''tilting level''' is a variant which has a precision vertical adjustment screw which tilts both the telescope and the high accuracy bubble level attached to it to make them level. This reduces the complete reliance on the levelling accuracy of the instruments's bottom mount, and the "split bubble" display gives additional assurance that the telescope is level whilst taking the sight. This allows faster operation as the bottom mount need not be truly level, though it will introduce a slight error as the vertical axis of the mount is not completely co-incidentcoincident with the telescope centre. The split bubble works by displaying half of both ends of the bubble side by side in the telescope, and when the curved ends are aligned it is level.

A '''digital electronic level''' is also set level on a tripod and reads a [[Barcode|bar-coded]] staff using electronic laser methods. The height of the staff where the level beam crosses the staff is shown on a digital display. This type of level removes interpolation of graduation by a person, thus removing a source of error and increasing accuracy. During night time, the dumpy level is used in conjunction with an auto cross laser for accurate scale readings.