Laser range finder for measuring bridge height

[danstanford]

I use a laser quite a bit in my job and brightness makes the beam hard to see but aiming it from the water (assuming it is not dead calm) would be impossible from that distance.

[Russ]

larrymac has the only useable idea it seems
I battle this problem in Florida where the bridge placards are very good but they assume MLW
I have to time my passage for low tide at certain bridges
I would like to hear more re the lasers.

My thoughts align with others in that we are talking inches of clearance, not feet or meters.
Standing on your deck alone means you need an Estimate of the height of the deck above the water ( not the at the water line of the boat) The bridge clearance is measured from the water obviously from the bridge plaquards)
Also where do you put the laser (even if you are under the bridge) the laser operator is standing up or lying on the deck The length of the laser unit itself is a few inches that alone would throw the whole measurement off it seems.
I'm French and kinda slow so maybe I don't see how the laser application would work even directly under the bridge.
So can someone with experience please explain the laser use more clearly? I'd love to use the concept.

[noelex 77]

Originally Posted by danstanford

I use a laser quite a bit in my job and brightness makes the beam hard to see but aiming it from the water (assuming it is not dead calm) would be impossible from that distance.

Laser rangefinders don’t use a wavelength in the visible range.

The instrument electronically registers the laser pulse and calculates the distance. You do not need to see the beam. It is possible to measure distances up to around 1.5km (1600 yards) or more in ideal conditions. Typical real world ranges are little more than half that.

[stimpsonjcat]

You could just do the math once for a fixed forward distance...say 100 feet to the bridge, horizontally. Solve for angle from your boat to the height of your mast at 100 ft, then just check angle to the bridge when you are 100ft away and as long as the bottom of the span is above that, you're good.
Let's see...
Angle= ASIN(<mast height in feet>/100)
So for a 50ft mast at 100 ft that yields 30 degrees.

Mount something at 30 degrees from the horizon someplace on the boat, or get a bubble level and a friend, and sight along that when 100ft horizontally from the bridge.

You could also solve for how long the hypotenuse would be at that angle, mount the rangefinder at that angle, and check the distance when the bridge crosses the rangefinder.

The signs seem a lot easier, if present. ;P

[Russ]

Apparently what we want for $400+ is this forestry gadget used to measure tree height.
Nikon Forestry Pro II
Laser rangefinder and hypsometer

[stimpsonjcat]

Originally Posted by Russ

Apparently what we want for $400+ is this forestry gadget used to measure tree height.
Nikon Forestry Pro II
Laser rangefinder and hypsometer

Shiny!

Just make sure you're in the right mode!

[Kolchac]

You are looking for an answer in a wrong chat. I read all three pages of answers, but no one gave a direct answer to the question!
So, I have three devices: 1- from Bosch range meter 150m, $200, 2-from aliexpress 200m, $40, both work this way: you direct the laser beam to the top of a construction/tree/whatever and it automatically builds the triangle including hight.
3- for golfers 1500 m, it has an angle measurement, I did not use it for measure the hight yet, only for the distance, but it should work.
Use for an answer in the construction and golfers sites.
Reply, if you cannot find, I’ll help.
For all skippers: if you do not know the answers, do not confuse people.

[JAFO]

Originally Posted by Bullshooter

(I couldn't do a superscript for "squared")

Online superscript generator: https://lingojam.com/SuperscriptGenerator

[Russ]

No offense Kolchac but I referred ya'll to the
Nikon Forestry Pro II
Laser rangefinder and hypsometer
Before you posted,you must have missed it.
Same concept as your devices when used in two-point mode
Tree/bridge same same
PS The Bosch unit requires a right angle between the boat and the bridge it claims
Measure distances "1" and "2" in this sequence with a length measurement. Pay attention that a right angle exists between distance "1" and the sought distance "E"

Can you tell me how you do this as this unit is much cheaper than the Nikon

[AlanCB]

Originally Posted by GinoDelG

Hi,
Is anyone using a laser range finder for measuring bridge height. If so, which model and how accurate is it?

We have tried it, using a range finder with height finding capability. We got heights within 10% of the charted height from distances where we could have easily abandoned our approach. That said, testing it, and depending on it, are two very very different things. You might want to get one and try it, just to see how comfortable you are with its estimates.

[Bucket]

We have done exactly this with a Bosch GLM 50 C. We took the dinghy off the ICW to a questionable bridge, and were able to measure the height of a bridge.

It was hard to find the red dot in bright sun, but once we did, it gave very consistent measurements. This particular model has an inclinometer, so in addition to measuring straight line distance, it can also be set to a mode where it uses the angle from vertical to measure the actual vertical height, taking into account the angle at which the laser measurer is held.

I believe this model can be found at Home Depot and on Amazon. The model numbers may change over time but this particular model was about $80 and has a Bluetooth interface option.

[Mickeyrouse]

I’ve rattled the VHF antennae on bridges a couple times, but don’t blame the charts. Blame the skipper- and once, an extremely high lunar eclipse tide.

[paul2884]

Originally Posted by lornolsen

You can NOT measure height with a laser range finder in the daytime. Just try measuring your mast. I have. It just doesn't work.

I use a laser meter for work and its very accurate INSIDE - as lornolsen said - it really doesn't like bright daylight, you might be able to do it at night but that might not be convienent or safe..

[requiem]

Originally Posted by Bucket

We have done exactly this with a Bosch GLM 50 C.
...
It was hard to find the red dot in bright sun, but once we did, it gave very consistent measurements.

That's a good point; there are a few different types of tools in play. I had forgotten I have a similar laser measure kicking around. One big difference is the sighting method. This type of measure places a visible laser dot on the target to help you aim; the hunting/golf/etc style has an optical sight that you look through instead. The latter is intended for outdoor use in bright sun and over much greater ranges, although the stated accuracy is much lower. I wonder if a spotting scope might make it easier to see the visible dot?

[bstreep]

We use laser measuring devices in our work every day. Leica (Disto) makes the best ones. The Bosch lasers are just not worth investing in. Plan on $5-600.

To measure, use an angle measuring device ($10) to determine when the bridge deck is at a 45 degree angle. Then measure to the base of the bridge. Add your height off of the water. That's the height of the bridge.

But here's the rub: That's just an estimate. I mean, it CAN be very accurate, but in practice, it's hard to do. I carry one of our "retired" ones, but I'd just stand off of the bridge and use binoculars to read the gauge.