Point of Beginning - April 2010 - (Page 46)
Dual-axis tilt-compensation systems.
Q: I really appreciate the modern total station because it takes care of so many things I used to worry about … like the two-axis compensation system that corrects leveling errors. But sometimes I wonder: Is there a downside to all these wonderful inventions? A: Modern total stations do have a number of routines to automatically detect and correct anomalous situations that are well within the normal range of error sources we encounter in surveying. But it is important to understand how they work and what they correct. The two-axis compensation system is a good place to start. First of all, it is called “two-axis” because it detects tilt in the two principal directions of the instrument’s local coordinate system. The x-axis is in the direction of the line of sight, i.e., wherever it is pointing in azimuth. The y-axis is normal to that; think of it as the “side-to-side” axis.
Y-Axis Tilt Detection and Correction
The purpose of detecting tilt in the y-axis direction (normal to the line of sight) is to correct horizontal circle readings. The reasons the small errors in leveling occur are similar to those discussed under x-axis tilt detection and correction. In addition to using the tilt in the y-axis direction, the inclination of the telescope is also used to compute the correction to be applied to the horizontal circle reading. Big warning: As you approach the limit of range of “out of tilt” in which the compensator will work, be aware that errors in plumbing over the ground point may be induced. If an instrument is not truly level, then the line of sight passing through the instrument
X-Axis Tilt Detection and Correction
The Geomatics Industry Association of America (GIAA) is a trade association of manufacturers, suppliers and distribution partners encompassing the present and emerging technologies in surveying, GNSS, engineering, construction, GIS/LIS and related fields.
www.giaamerica.org Regular Members: Leica Geosystems Inc. Topcon-Sokkia Trimble Engineering & Construction Division Associate Members: POB magazine The American Surveyor magazine Professional Surveyor magazine Affiliate Members: USACE – Philadelphia District
The purpose of detecting tilt in the x-axis direction is primarily to determine the correction to be applied to the observed vertical circle reading. The vertical circle is oriented so that zero degrees on most instruments is along the upward direction of their mechanical vertical axis. The process of leveling the instrument is to align that mechanical vertical axis with the true vertical—the direction of gravity. If the mechanical vertical axis is slightly out of plumb when a vertical circle reading is taken, then the error in the vertical circle will have a component that is exactly equal to the out-of-plumbness in the direction of the line of sight. (There are other errors possible when a vertical circle reading is obtained, but we won’t get into them here.) The mechanical vertical axis may be out of plumb for several reasons. The plate bubble may be out of adjustment; the ability of a person to center the bubble has limitations; the instrument may have settled slighty after intially being leveled; solar radiation, wind or wind-caused vibration or cooling may be causing movement; or expansion or contraction may have occurred in the tripod or even the instrument itself. These and other incidents can induce small leveling errors.
With a 5 arc-minute error in the mechanical axis, the plumb line through the optical plummet is shifted 0.006 feet.
APRIL 2010 | Point of Beginning | www.pobonline.com
Table of Contents for the Digital Edition of Point of Beginning - April 2010
Point of Beginning - April 2010
A Grand Re-Entrance
A Model Community
Tough Work in a Tight Spot
Show Me the Data
BIM in a Box
Traversing the Law
New & Notable
Point of Beginning - April 2010