March 23, 2012 at 11:59 am #158104
Andrew Garulay, RLAParticipant
Last summer I posted a thread looking to see what others use for equipment when they need to field locate things that were either missed on a survey or in lieu of a survey. I know that few of you actually do this, but I thought I’d share what I discovered to be a very useful tool.
It is called a double right angle prism (mine is a CST Berger). Only about the size of a film container (or a prescription pill container for those under 30), it is a precision device which allows you to view 90 degrees to the left, 90 degrees to the right, and straight ahead (perpendicular to the other two views) all at the same time. The three views are stacked in a little window on the device.
The top image shows 90 to the left, the bottom shows 90 to right, while the middle shows 90 degrees to that line (or straight ahead). If you place two takes in the ground (measured of a building for example) you can look in the prism as you line yourself up in between them. When the two stakes line up in your view, you are on that line. Then you can move left or right to line up the target that you want to measure in that middle window. When all three are aligned you can accurately assign coordinates by measuring to the target and to one of your stakes. I use a laser range finder (Bosch DLR165K) to do that, so I don’t have to move.
More often, I use just two of the views. I line myself up with an end wall of the building so that the front and rear corners are lined up. Looking at the middle view in the prism (straight ahead), I check to see that I’m in line. Then I look in either of the other prisms and walk forward or backward until an object (usually a tree) lines up with the house corner. When it does, I shine my laser on the building corner for one coordinate and on the tree for the other and write it on the clipboard sketch that I carry around. I use a lanyard to hand the rangefinder around my neck so that I don’t have to put things down and pick them up.
Back at the office it is just a matter of offsetting a couple of lines, using a zero fillet, and popping in a plant symbol with precision.
I don’t use the prism every time, but it works great when I need it. That is usually when I can’t measure from an already located object for whatever reason (too much vegetation, trucks in the way, …). Then I use it to layout a set of stakes which I can then use to measure from.
I know most of you get amazing surveys and don’t need to do this, but for the other two that do some site measuring, I hope this helps.
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