Commit To Cut – Photogrammetry Guides the Saw

A good friend of mine was planning to sort his back garden out, but life threw stuff in the way and the project stalled. Built on a slope, right outside the back door was the planned space for a set of steps that lead into the rest of the garden. Could we design and make a set of steps and get the project moving again?

Breeze blocks and bricks filling in for some proper steps.

Scan It!

My friend lives 100 miles away. Having enough information to build a set of bespoke steps could need a lot of measurements. Or we could shoot a hundred images and build a 3D model. Adding a few key measurements to the model should ensure accuracy of the whole model. Could photogrammetry help design and manufacture the steps remotely?

Metashape model of the step recess with points and scale bars added.

With the model built in Metashape Pro the key points and scale bars added. My friend took a few arbitrary measurements, proving the scaling applied to the model worked. At this point it would have been really handy to export the model in a format the CAD system would import, but this was not to be.

CAD Time

A lifetime ago I specified a 3D CAD system for a company I worked for. In 1995 a single seat of 3D CAD software and hardware cost £20k. Nowadays there is free 3D CAD software available and it will run in a web browser. How times change. Oneshape is such a CAD tool and costs nothing for a free account so the first job was to model the shape of the step recess.

Screenshot of step recess (in red) with sleepers placed


The garden already had some raised beds made from reclaimed railway sleepers. Steps made of sleepers would match nicely and are pretty cheap. First step (no pun!) was to position the sleepers into the recess. With all 5 steps positioned the step supports could be modelled to fit. Onshape has a neat boolean subtract command that lets you fit a complete sleeper into position and then use the steps themselves to cut the profile into the support – and if you move the position of the steps slightly the support would update itself – very neat and quick.

Step supports – stringers – modelled in-situ to support the steps

The step supports were made from sleepers (keep material choice simple!) and these parts were the most complex parts needed. A 2D drawing was generated.

2D drawing of the step supports. Two of these parts were needed.

Cutting Time

There were two problems when it came to making the steps; the garden was 100 miles away and a full length railway sleeper wouldn’t fit in the car. With limited access to the back garden the reclaim yard who had the sleepers wouldn’t deliver…so the answer was to cut the parts offsite, and once trimmed to size all the sleepers would fit in the back of the car – problem solved! As long as the measurements were accurate everything would just slot into place…

Step supports cut and fitted.

The step supports went in first and needed no trimming. A quick verification of the step dimensions helped with the confidence levels. When designing with 3D CAD its worth bearing in mind the software is a precise tool and everything was designed around a 10″ (152mm) sleeper width. In practice the sleepers varied in width by up to 1″ (25mm) so each sleeper was carefully selected and cut with a chainsaw at the reclaim yard.

The Sleeper Chainsaw Massacre

Precision cuts with a chainsaw takes care and when it came to fitting two of the sleepers slid straight into place. The rest needed a little fettling with a plane to ensure a snug fit but nothing was undersize or wasted. The lower step was the most complex, having to fit over the brick sill, so this one was trimmed onsite.

Precision work with the chainsaw

Everything fitted as expected. The photogrammetry model was a constant reference, verifying any dimension we wanted as the design emerged from 3D CAD.

The Result

The steps fitted a treat with minimal tweaking and no scrapped timbers. A highly satisfying job that proved the value of photogrammetry. The only request would be for Metashape to export a 3D format supported by Oneshape, but from previous experience its known 3D models with a lot of facets/faces can cause 3D CAD performance issues so its not a guaranteed improvement to the process.

The finished steps.