To what extent can two EDM techniques be used to record

archaeological artefacts at sites where time and access are

restraints on surveying?

By

James Tidswell

Ocean Exploration Stage 3

Project supervisor: Ken Kingston

Project aims and objectives

The aim for this project was to;

• To compare the accuracy of two electromagnetic distance measurement techniques (EDM) used in surveying archaeological features and sites such as Blaxton tide mill.

• Determine the most appropriate technique.

Blaxton Tide Mill

Site selection

• Decaying historic landmark in close proximity to Plymouth

• The site is tidal = time and access are critical factors to be taken into account when surveying

• Half of the site is submerged at high tide preventing access -> time and access restrictions.

• Performance and results of the surveys can be used for other estuarine archaeological sites

Total Station Dimcon Survey • Dimensional control Survey

• Leica TS02+ Total Station

• Control network set up around the site

• At each setup: • Shoot in control points first (4 common CPs for each).

• Shoot in features of interest e.g Walls/arches

• Close the survey

• Move to the next location and repeat

• Processed in SC4W dimcon software.

Source: Google Earth

Terrestrial Laser Scanner (TLS)

• Setup similarly to the Total Station survey

• Targets set up around the site at strategic positions.

• 4 targets visible from each setup.

• Scan in the targets, select range, set limit angles and start the scan.

• Scan resolution 1.2mm at 10m

• Processed in Leica Cyclone and SC4W

Screenshot from Eiva NaviModel Viewer

Positioning

• Post-processed static GPS taking differential corrections from the PMTH OSNET base station

• Converted from ETRS89 lat/lon to BNG Eastings/Northings via GridInquest

• Baseline created between CP 3 and 4 • Provides the bearing for processing in SC4W.

• Logging for 1+ hours as defined by RICS guidelines for surveying – less than 20km from the base station.

Results – Technique Comparison

Comparison Criteria TS survey TLS survey

Instrument and software expenses

(Cheapest) ✓ x

Accuracy (best) - -

Time Consummation - Fieldwork

(Least)

x ✓

Time Consummation - Processing

(Least)

✓ x

Modelling Capability/ PC density

(best) x ✓

Equipment Expenses

Equipment Cost

Instrument Cost (at time of purchase)

Additional Costs (software)

Leica TS02 total station

£6,250

Large range or software available. SC4W

£1500 (educational discount) £3000 (without discount)

Leica P20 Scan station

£60,000 (incl. educational

discount)

Leica Cyclone £200 per module (educational

discount) £9000 per module without

Accuracy

Instrument Accuracy

Instrument Range Accuracy Angular Accuracy

Leica TS02 TS 1.5mm + 2.0ppm 3" (1 mgon)

Leica P20 TLS 1.2mm + 10.0ppm 8” (2.46 mgon)

Source: Leica Instrument manuals.

1mgon = 0.0009 degrees

Time consumption - survey

Time sheet for Leica TS02+ Total station

Activity Time taken per set

up (minutes)

Setting up control points and prisms

15

Setup total station at position

5

Shoot in the control points

15

Shoot extra features average time

30

Export data to memory stick

2

Setting up equipment at each setup

35

Total per setup (including target setup

and moving) 65

Time Sheet for Leica P20 Terrestrial laser scanner

Activity Time taken (minutes)

Setting up targets and tripods

15

Scanner set up (including start up)

5

Naming, shooting and storing the targets in the

scanner

15 (per setup - 5 targets)

Scanning and imaging– average time

26.4

Exporting data to memory stick (excluded

in total per setup) 45

Setting up equipment at each setup

35

Total per setup (including target setup

and moving) 61.4

Consistent with Chekole (2015) result of a TS vs TLS study.

Time sheet for Laser Scan processing

Activity (using Leica cyclone unless otherwise stated)

Time taken (minutes)

Obtaining positions of Targets via SC4W

30

Creating database and importing ScanWorlds

120

Creating control ScanWorld and applying target

coordinates 10

Scan Registration 10

Scan cleaning 120

Basic mesh production 10

Total 300

Time sheet for Total Station processing

Activity Time taken (minutes)

Obtaining point positions through SC4W

60

Correcting for height of prism staff

10

Total 70

Time consumption - processing

Point cloud density Survey Type Number of data points

Total Station survey 319

TLS Survey 300,162,492

Point cloud comparison

• Carried out in Cloudcompare

• Distances between point clouds calculated and histograms obtained.

• Differences of up to 0.01m between the clouds

• Minimum spacing 0.0013m (1.3mm) • Within stated instrument accuracies

Approx. 8 points Approx. 1037000 points

Point clouds used for PC comparison

Screenshot from ArcGIS

1896 OS Map – POST DESTRUCTION

Blaxton Tide Mill

• Flour and Grist Mill located on Blaxton Creek

• Constructed between 1830 & 1856

• Destruction occurred between 1856 and 1896

• Technology is a precursor to modern tidal renewable projects

What do the results tell us?

• TLS provides significantly better coverage - 300million vs 300

• TLS and software is expensive

• Time consumption depends on a variety of factors • resolution, angles of scanning, range filters, how many setups and

how many targets/points

Conclusions

• TLS is the best option due to the volume of data density in a short amount of time.

• Decay of the mill and decline of the area most likely due to increased industrialisation combined with termination of lease

References

• Kleinot, J. and Househam, I. (2015). Monument recording and monitoring form - Blaxton tide mill, Quay and lime kilns.. Tamar Valley AONB.

• Wailes, R. and Gardner, E. (1956). Tide Mills. 1st ed. Society for the Protection of Ancient Buildings.

• Chekole, S. (2014). Surveying with GPS, total station and terresterial laser scaner: A comparative study. [online] Available at: http://www.diva-portal.org/smash/get/diva2:715829/fulltext01.pdf [Accessed 29 Apr. 2016].

Questions?