Laser scan

I’m being slow at organising my data, but today I managed to re-sample the NMI Carolan harp laser-scan, and uploaded it to Sketchfab. This allows it to be easily embedded in web pages.

The Carolan harp (National Museum of Ireland) by Simon Chadwick on Sketchfab

I’m releasing this scan as CC-BY-SA which allows anyone to use and modify it however they want, as long as they credit me and release any derived works under the same license.

Sketchfab does allow downloading of the low-res model but I have also made a link to the full-res version: OBJ mesh file. I use the MeshLab app to view and manipulate the OBJ mesh file

Here’s the video of Elaina Sugrue of Accuscan, making the scan back in the October 2018. You can see how the point-cloud, captured by the scanner, is rendered real-time on her laptop screen. This scanning process generates a huge amount of point data, which had to be processed, and separate passes with the scanner “registered”, to generate the finished mesh file.

I think it is important to be able to release this kind of primary data, as part of the project to understand the old harps more. This scan is a wonderful resource, but it needs a lot of further study to be of practical use. I have made many slices and renderings, which in due course I will publish.

This harp, being very damaged and distorted, requires also a lot of theoretical reconstruction work. Hopefully in time we can also publish reconstruction drawings. I am still thinking about how best to go about this.

I remembered my old post, Archaeological copies of old Gaelic harps from back in 2016. We are not moving at the rate I suggested of one per year, but this kind of study and documentation is an important part of this kind of long project.

The header photo is by Brenda Malloy, and shows myself and Elaina Sugrue at the National Museum of Ireland in October 2018

Laying out the pins on the neck of the harp

In 2011, Michael Billinge wrote an interesting observation on the layout of the tuning pins on the neck of the Downhill harp. Talking about the way the tuning pins become more spread along the cheek band, as the angle of the neck becomes higher in the bass, he writes: “instead of an even change across the range, as might normally be expected, he seems to have done this in a series of blocks or groupings”

In his footnote, Billinge gives a list of the gaps between adjacent pins, and the way that they increase in steps. My chart below gives a visual representation of his data:

Pins spacing on the neck of the Downhill harp (mm) (data from Michael Billinge)

What made me think of this was that I was doing the same work this week on the Carolan harp. I tagged points on the laser-scan corresponding to the cheek-band holes in the left side (string side) cheek band, and then calculated the distance between each one. Plotting a graph of these distances showed clear groups of similarly-sized spaces.

Tuning pin spacing on the neck of the Carolan harp (mm)

Billinge does not say what the error margin on his measurements are, and so it is difficult to analyse them further. On the Carolan harp, the error on the picked points is less than 0.1mm, but the selection of what points to pick is much less accurate than that, since the scan is quite messy around the tuning pin, with lots of scanning artefacts. I would estimate the accuracy of my measurements as perhaps ±1mm

You can see on my graph that there is a certain amount of zig-zagging, alternating around an average value. I saw this also on the spacing of the string shoes on the belly, but I explained that as an artefact of the alternating shoe design. I’m less sure how to understand this alternating spacing on the neck.I have not done such a detailed measurement of any of the other harps, but the point positions of the tuning pins as used to generate the string charts for the Kildare and the Mullaghmast harps can be used to analyse the pin spacing. The accuracy here is perhaps more like ±3mm. The Kildare seems to show some evidence of grouping, but the Mullaghmast pins are clearly spaced incrementally, with each pin a little further from its neighbour than the previous one.

Pin spacing on the neck of the Kildare harp (mm)


Pin spacing on the neck of the Mullaghmast harp (mm) (cropped, the largest spacing is 116mm)

We could follow Billinge by averaging each group on the Carolan harp, and calculating a standard deviation from the average:

Pins 1-3: 17±1.5mm
Pins 3-6: 14.5±6mm (too erratically placed to say much)
Pins 6-12: 13±1.5mm
Pins 12-19: 14.5±1.5mm
Pins 19-26: 18.5±1.5mm
Pins 26-30: 22±1mm
Pins 30-32: 26±1mm
Pin 32-33 crosses the opened neck-pillar joint.
Pins 33-36: 28.5±1mm

I think this kind of analysis can give us ideas about the working methods of the old harp makers. We can imagine the makers of the Downhill and the Carolan harp, working with dividers to lay out groups of pinholes on the metal cheek-band, as well as using dividers to lay out the string-shoes equally spaced on the soundboard.

What then of the Mullaghmast harp, with its progressive spacing? A different school of harpmaking?

My header photo shows a rendering from the laser-scan, showing two points picked for holes 9, 8, 7, 6, 5, and 4. The position of each hole was calculated as an average of the positions of the two points. The background grid is 1mm x 10mm. The messy damage around hole 3 and the break in the neck is visible in the top-right corner.