Total station horizontal angle/vertical angle ??

Instrument adjustment issue.

Your instrument is more than likely applying the correction for the tilt axis error.

However it is something to look at and understand.

If you want to be looking at a specific angle, the display needs to show that angle - Adjust until the angle displayed is correct - if your accuracy requirements demand it.

If the instrument was carefully leveled before turning the angle, then there is an adjustment problem in that the axis of the vertical motion is not horizontal. Careful leveling includes rotating it to see that the fine bubble doesn't move in its tube.

To get accurate results in this condition, take the average of the angles measured with the instrument direct and reverse, also known as face 1 and face 2. You may get 89-59-45 and then flip it and measure 90-00-15. The display will not be right in either case, but the average will be pretty good.

If you really have to be gnat's, lay it out direct and reverse face, then split the difference. This may be especially visible the steeper your vertical angle. When considering your error budget, remember that 20 seconds is 0.01' in 100'.

Think about it this way. All inst. have some amount of error in their vertical axis. Imagine it exaggerated, instead of the cross hairs traveling perfectly vertically following a plumb line, they are moving to the left as it goes up. An older theodolite would show 90-00-00 no matter how far to the left of the plumb line the crosshairs moved.

Newer total stations utilize a columniation routine to measure know how far off the plumb line the cross hairs are at any given vertical angle. They correct the horizontal angle to correctly represent the true horizontal angle.

So if you really need to be sighting exactly 90-00-00 (horiz. angle) you have to re-adjust. For many stakeout activities using a properly adjusted instrument, the tilt axis error is not critical.

EDIT: The Trimble 5600 series instrument advises that the collimation routine should be carried out immediately prior to measuring high precision angle measurements. They refer to it as "trunnion axis tilt"

Like many of the others on this page said, this is your horizontal tilt sensor kicking in to adjust for the instrument being out of level. Make sure you level the instrument as closely as possible.

There is a tilting axis adjustment for this that will take into account your double centering error, tilt error and plumb line adjustment and keep this from happening.

Most surveyors just turn off their horizontal tilt sensor and never see this.

When considering your error budget, remember that 20 seconds is 0.01' in 100'.

That's a good rule of thumb that I like to use. I always think of it as 1 second is 0.01' at 2000'. Not perfect, but it comes in handy.

You are correct in adjusting the angle back.
Don't turn it off, it is smarter than we are, or at least smarter than me. Your accuracy will go down.
Can you imagine calculating the angle based on how much it's out of level?

> Most surveyors just turn off their horizontal tilt sensor

I doubt that, why would they?

As others have replied, if the instrument drifts away from your desired horizontal angle reading as you elevate or plunge the scope, the horizontal compensator or trunnion axis compensator is correcting the horizontal angle being displayed to compensate for the out-of-level condition.

To test this for yourself, suspend a plumb line in a place convenient to sight with the total station. Set up a few feet away, say 30 feet or so, and level the instrument as carefully as you can, taking care that two leveling screws are parallel with the line from the instrument to the plumb line. Sight the plumb line as low as you can see it and set the horizontal angle to 0.

With a properly adjusted and leveled instrument, as you elevate or plunge the scope, the crosshairs should follow the plumb line. So to check how well your compensator is doing its job, orient the instrument so that you can purposefully take the instrument out of level perpendicular to the line of sight to the plumb line, while keeping the vertical tilt sensor readout at 0°00'00". You will need to be in the screen that allows you to see what the tilt sensors are reading to do this. Most conventional total stations have a tilt range of 3 minutes. Adjust it so that the horizontal sensor is out of level the full limit of the range.

Then 0-set on the plumb line at the lowest point you can see. Then note where the crosshairs are looking as you elevate the scope. If the left side of the instrument is low, the crosshairs will move left of your plumb line as you elevate the scope. The further you elevate the scope, the more the horizontal angle will change.

On a properly adjusted instrument, when you turn the collimation back to the plumbline, the horizontal angle should return to 0 or whatever the angle that was displayed when you sighted the plumb line as low as you could see it.

Short answer to your initial question: yes you were correct to adjust the horizontal angle back to what was desired. But if it was very much, your instrument was whispering to you that it was out of level. If the plate vial indicated that the instrument was level, it is in need of adjustment.

So really the horizontal angle changing while the vertical angle is adjusted is the result of the axis compensators correcting for the instrument being slightly out of level, then additional correction is applied for trunnion axis error measured through the columniation routine.