GIAA Mailbag: On Tribrachs and Phase Error with Targets
A: The instrument rests on three points of contact. The wobbling or “rocking” you experience is due to wear on the instrument’s base or the contact points, or movement of the contact points on the tribrach. Such looseness can accelerate wear. More importantly, accuracy in leveling, centering, and angle and distance measurement can be compromised. Several methods are used for constructing tribrachs, but on most the three contact points are hardened steel screwed in from beneath the tribrach. If they are hardened steel, the wear may be minimal, but it may be possible to adjust the fit by turning the appropriate screws. On lower quality tribrachs, the contact points are cast and are an integral part of the base. If wear has occurred, replacement of the tribrach may be the best solution. Wear of the instrument’s base, if minor, can be handled by adjusting the tribrach contact points. In extreme cases machining or replacement of the base may be necessary. As you use a set of tribrachs, adjusting one part of the system may cause incorrect fit with other parts. A complication to this is the mechanism used to lock the instrument (or target) into the tribrach. On many tribrachs, as this mechanism pulls the instrument or target down onto the contact points, it may cause a slight rotation of the instrument or target. The best solution is to take it to an experienced repair technician who can do the machining. Take all of your component parts and adjust them all so you are assured of uniform fit, ensuring accurate location of the total station and targets in the tribrachs.
Q: We hear about “phase” with respect to GPS and with EDM. But I recently read about phase error with targets. What is this and how (if it is possible) do I eliminate it?
A: Phase error with targets most often relates to apparent movement of the object being sighted for angle observations due to a combination of the way the target is constructed and lighted. The most common occurrence of phase error occurs with the use of prism or range poles for targets. Sun or different intensities of lighting from different directions can cause part of the target visible to the observer to be in shade or partial shade. As the distance between the instrument and the target increases, the lit part of the target or the shaded part of the target, depending on observing conditions, including background, may become so dominant that the observer erroneously aligns the cross hairs with the apparent width of the target rather than its true width. This makes it so that they are not collinear with the vertical line passing through the ground point being observed. This error could happen because the shaded part or the lit part disappears, or it could happen because the darker or lighter part of the target makes it appear smaller (and unknown to the observer), off-center from the true line. It used to be very common to use a pair of flat boards attached at right angles to each other (so that in plan view the edges of the boards formed a cross) to improve the characteristics of the target being observed. Today, the most common method of eliminating this problem is to use targets similar to the so-called traverse targets that are flat, with triangles, diamonds or circles to denote the optical centers of the targets.
GIAA will be sponsoring a workshop on "how things work" at the 2003 ACSM-APLS conference. Join Joseph V.R. Paiva, PhD, PS, PE, in this four hour workshop covering the basics of surveying technology to help the surveyor better understand how to maximize performance.
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