Measuring tape for GNSS tripod

Disclaimer: DO NOT USE THIS FOR LEGAL SURVEYING

Inspired by the Leica GZS4-1, it doesn't require a dedicated tribrach or adapter. Will work with most surveying tripods, but is designed to fit snugly on a Leica-style tribrach and a Leica GST120 tripod.

Parts

The thing comes in 7+ parts: 

• 2 body halves
• 1 tape reel cover
• 2 half dowels for the coil spring mechanism
• 1 mounting collet
• 1+ spacers for different GNSS pole diameters

Printing

• You need to use a material with a good mix of stiffness and durability. It works well with PLA but it might distort over time. Carbon fiber PC blend would be a good option for longevity
• For a better look and perfect mating surfaces, print with the body faces that will glue together on the build plate
• Print the reel cover with the mating surface on the build plate
• Supports are needed in the reel cavity on the back body half and the reel cover
• Try to print the half dowels laying flat on the build plate. They work fine printed standing though if you're having trouble making them fit in the body
• You might have to tweak the spacer size by scaling in XY to account for slight variations in printed dimensions. There needs to be as little play as possible between the collet/spacer/pole

Additional parts

Cheap tape measure

You will need a small 12'/3.5m tape measure or shorter length. I designed around this particular one. Other potential options: This, this, or this. Cheap tape measures from different brands might be built around the same internal parts or very similar.

The important dimensions to adhere to:

• The tape must have a maximum width of ½" (12.7mm)
• The spool diameter must not exceed 46mm
• The opening in the spool for the spring stud must be at least 12mm

Double-check that it actually measures accurately before using it!

Fasteners

You also need:

• 3x M3x16mm or 18mm flat socket screws
• 3x M3 hex nuts
• 4x short pieces of 1.75mm filament as alignment pins
• A rubber elastic to keep the tape from uncoiling while you're working

Assembly

(next picture is a short stubby test version without alignment pin holes)

1. First, you need to glue the body halves together:
   • Put the alignment pins in and the screws halfway in on the back half (not the one with the thin part at the tape end) before bonding the parts so everything stays aligned
2. Then, insert the half dowels in the body.
   • They don't need to be bonded in place. In fact, if they ever break, you want to be able to replace them
3. With pliers, wind the coil spring a few turns so it's under tension
4. Insert the tape reel in the body keeping everything from uncoiling catastrophically
5. Making sure the half dowels are properly aligned, pop the reel cover over the body
   • You can test the tape retraction now. It should snap all the way in. If not, take the reel out and wind the spring a couple more turns
6. Take the screw out from the back, put the nuts in and insert the screws in the front until everything is secure.

Use

• Use the spacer that fits your GNSS antenna pole diameter to slip the mounting collet to the top of your tribrach adapter
• Hook the bow into the dovetail on the collet
• Extend the tape to the ground point and read the tape at the reel mouth.
• Add 210mm to get the height to the top of the tribrach adapter.
• Your ARP height will be: measured value + 210mm + length of your GNSS pole