Topcon GR-5 static session H.I. reference point

Should be the BASE of the unit (see link below):

Loyal

The manual for the unit should have a little sketch with dimensions so that you can make up an Excel worksheet to calc the ARP height from the slant height reference point.

Or if the receiver is on a regular tripod then just use your pocket tape to measure the height to the base of the receiver mount. Doing this you might lose a millimeter or two but usually that isn't critical. I measure it in meters and feet, enter the meters into the controller with an M after it and the controller will convert it to USFeet (if your project is set to USFeet), that's my check.

I have done both.

Or you can get one of those big gangly, aluminum, fixed height tripods and wrestle with that thing (including adjusting the bubble constantly) only to have it expand in the sun and lift the point off of the monument. [sarcasm]But I am a Surveyor and know how to measure things![/sarcasm]

I don't know about the GR-5 but on the GR-3 it's older twin the ARP is the bottom of the unit as shown in the NGS sketch. There is a reference point on the side of the antenna collar that is used with the Topcon Data Collector software and Topcon Tools.
There is a diagram inside the battery cover of the unit that shows the ARP.

A fixed hight (2M) rod works well HOWEVER after using the unit for a while I finally realized why the large, expensive, heavy, cunbersome GPS tripods exist. Watching the GR-3 (a rather heavy unit) whip aroung in the wind when mounted on a 2M carbon fiber rod tells me that the GPS tripods are much more stable, even if they are a pain to transport and set up. I guess if you are near a road, or can drive to your site, they may be the thing to use. I do use a regular tripod a lot, that seems to work the best just measure carefully, twice.

T.W.

We use fixed height tripods when possible, but for our old Ashtech Z12 units, I did an
Excel spread sheet that converts the slant height measured to the top of the ground plane to a vertical height of the ARP. I set it up so that it printed all of the HIs in 1 mm increments with 1 decimeter per page. We keep a copy of that booklet in the case with the receiver and correct for the height to the ARP, when we measure up. My table goes from 1 meter to 2 meters.

We have been doing a lot of observations for OPUS Projects, and having the same measure up allows us to bulk load RINEX files that are zipped.B-)

> Or you can get one of those big gangly, aluminum, fixed height tripods and wrestle with that thing (including adjusting the bubble constantly) only to have it expand in the sun and lift the point off of the monument.

In hundreds of fixed-height setups, I've seen two or three that ended the session with the pole tip slightly off the point. Whether they started the session that way is a open question.

Setting up a 2-meter tripod only takes a few minutes. Yes, they're long -- they'd be a pain to thread through dense woods -- but not particularly heavy, and they shoulder easily. The bubbles can be finicky to keep in perfect adjustment (they're 8- or 10-minute bubbles, typically), but since you can rotate the center pole while set up it's not a big deal. (Adjusting the bubble makes a good activity for long static sessions.) They *are* expensive, which keeps many from acquiring them.

For small networks with only a few operators, carefully-measured setups with conventional tripods are fine. But when you have many days of 10-receiver sessions, chasing down suspected HI blunders becomes way too frustrating, and fixed-height is the only way to go.