to what extent can two edm techniques be used to record … · 2017-05-30 · leica ts02 ts 1.5mm +...
TRANSCRIPT
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?