Today's setup. I can't find the picture I posted for this same location last fall when things were brown.
I like the 2.000/2.500 meter pole I made out of some telescoping thing I found at a recycling store. I carefully filed holes for pins to select the height. So much simpler than the measure-up process for elevations, but horizontal centering is terrible because I don't have a precision level rigged to plumb it yet and can only expect within 20 minutes or so of plumb.
I'm using a level held along each side of the pole, but it isn't sensitive enough. For GPSonBM the horizontal position is secondary, so I thought the pole a decent tradeoff to remove the opportunity for blown measure-ups. And I have a particular disk I want to measure right next to a 2.5-meter-tall shed so needed a pole.
I have acquired a sensitive bullseye level bubble but didn't spring for a manufactured assembly so am still figuring out how to mount it to a piece of angle stock that I can hold to the pole.
The rod level has a 40min vial and is useful for many tasks from aligning posts, monuments, prism poles and other vertical needs. They cost from $7 to $20 depending upon the supplier.
RPLS NE Texas
40 minutes is about what I've got and hope to read to half of that. But 20 minutes is 14 mm at 2.5 meters, and on a good site the expected rms horizontal error in a 6-hour solution is about 5 mm. It doesn't seem acceptable to have the centering error 3 times the GPS error, hence the need for a disclaimer note or a better level.
The rating on the bubble is either per 2mm of movement or as marked. A 40 minute bubble of the 2mm variety should be able to be read to 1/50th of an inch or about a half mm.
Centering a bubble within 1/50th of an inch isn't as easy as it sounds, in my experience, even when using a bipod or tripod. I long ago junked my 40-minute bubbles in exchange for 8- or 10-minute vials, with a very noticeable improvement in results.
Frame Surveying & Mapping
609 A Street
Davis, CA 95616
The average human eye detects vriations down to 1/50th of an inch. When the bubble looks perfect to you it should fall in that nominal range. It is not difficult to bring a 40 minute bubble into apparent perfection. The pole will move about a hundredth per 10 minutes, which equates to the 1/50th of an inch error in our visual abilities.
It's worth noting one of the configurations that improves that ability is aligning concentric circles. Also worth noting this is only one source of error. We all put the bubble in precisely the wrong spot for a number of reasons.
CFedS, PLS ID-OR-WA-UT-NV
Exactly. The primary purpose of my sessions is for elevation for GPSonBM, so avoiding measure-up is desirable.