Skyline, azimuth and altitude tools for archaeoastronomy

Skyline tools and theodolite Apps
for archaeoastronomy
Skills, Sep. 14th, 2016
Victor Reijs
victor.reijs@archaeocosmology.org
http://www.archaeocosmology.org/eng/skylineprofilescomparing.htm

Knowing the (apparent) altitude is essential
for latitudes >40°
Parameter (device) Parameter
StDev in deg
Impact Resulting declination
StDev in deg
Latitude (GPS) 0.01 0.5 0.005
Altitude (clinometer) 0.2 0.9 0.18
Azimuth (compass) 0.5 0.5 0.25
Resulting 0.31
• At average altitude of 1°
• At average latitude of 53°
• Azimuth between 30° and 150°
http://tinyurl.com/j8myvfb

Part 1
• Tools to determine skyline
• Apps for augmented reality - theodolite

Tools to determine skyline
• Desk based research
– height grid
– 3D model
• Field based research
– photometry
– theodolite (compass and clinometer)

Some height grid based tools
• OS map
– Manual (Alexander Thom)
– Horizon
standalone programme since 2002 (Andrew Smith)
• SRTM (Shuttle Radar Topography Mission) grid
– 90m resolution
• HeyWhatsThat
Web service since 2000 (Michael Kosowsky)
• Horizon
standalone programme since 2002 (Andrew Smith)
• SyntH
privately run service since 2011 (Fernando Patat)
– 30m resolution
– SyntH
privately run service since 2015 (Fernando Patat)

Ins and outs of
height grid
• Can be low resolution
• Large errors for nearby objects
• Can have gaps/voids in the grids
None of the tools depict the documented SRTM voids
(height value -32768)
• Does not include plants/constructions at horizon
• Fixed position

Comparing different skyline programs
Theodolite
SyntH
SRTM 30m
Horizon
SRTM 90m
HWT, SyntH
SRTM 90m

Missing grid data
SRTM 90m
SRTM
Missing grid data
Photo by M.J. Zawaski and J.McKim Malville

Some additional features of
grid based tools

HeyWhatsThat skyline in Stellarium
http://www.heywhatsthat.com/api/horizon.csv?id=<HWT-ID>&resolution=.125

HeyWhatsThat viewshed in Google Earth
Common visibility in Google Earth

Ins and outs of
3D model
• Based on height grids
• To include terrestrial refraction can be
cumbersome
• Flexible position

13
Ring of Brodgar, Watchstone, Standing stones of Stenness, Barnhouse Stone, etc.
Based on 3D model
VRML and OS map
Video by Victor Reijs: NAM2003

Based on 3D model
VRML and OS map
Video by Victor Reijs: NAM2003

Ins and outs of
photometry
• Stitching-, level- and/or lens distortion errors
• Possible influence of atmospheric haze
• Includes plants/constructions at horizon
• Fixed position

Based on photometry
SRTM
Photo
Photo by Michael Wilson: http://mega-what.com/

Ins and outs of
on theodolite
• Time consuming
• Depending on weather
• Possible influence of atmospheric haze
• Fixed position
• Is quite accurate (seen as benchmark)

Based on theodolite
Theodolite
SRTM
Theodolite data by Michael Wilson : http://mega-what.com/

Conclusions Part 1
• Know your tools!
• The higher the resolution the better
• A combination of skyline tools is essential
• Provide constructive feedback to tool builders
• Verifying with reality is essential

Part 2
• Tools to determine skyline
• Apps for augmented reality - theodolite

Altitude - clinometer

Azimuth - compass

Augmented reality - theodolite

SunSurveyor

By Alison Chester-Lambert: 25 June 2014
Portrait position
The Circus, Bath: summer solstice
Summer solstice
path
Sun’s position
on 25/6/2014

Difference in measured azimuth
Portrait up and down

Landscape position
The Circus, Bath: summer solstice
Summer solstice
path
Sun’s position
on 25/6/2014

Difference in measured azimuth
Landscape up and down

Error in averaged azimuth
(four positions)

Error in averaged altitude
(four positions)

Conclusions Part 2
• Azimuth error does not depend on App, but on
device (soft and hard iron influence)
tested: Samsung Tab2, iPad Air 2 and iPhone 6s Plus
• Azimuth StDev of my Samsung tablet (>2.8°) is
much larger than compass (>0.5°)
• Altitude StDev of my Samsung tablet (>0.2°) is
somewhat larger than clinometer (0.2°)
• For nearby targets: watch out for parallax error
camera is not in middle of device

Conclusions
1. Getting a sustainable skyline (web) service
might be important for archaeoastronomy
discipline!
2. Smart devices are not yet (mid 2016) mature
enough for accurate azimuth measurements.
Provide constructive feedback to tool builders

Questions? http://tinyurl.com/j8myvfb

Selected of sources
Kosowsky, Michael. 2012.'HeyWhatsThat', in http://www.heywhatsthat.com/ [accessed 10 March
2012].
Patat, Fernando. 2011. 'Horizon synthesis for archaeo-astronomical purposes', Astronomische
Nachrichten, Vol 332: pp. 743-49.
Reijs, Victor M.M. 2001.'The reappearing sun at Orkney', in
http://www.iol.ie/~geniet/maeshowe/eng/flashing.htm [accessed 4 March 2012].
Reijs, Victor M.M. 2007.'Refraction: Refraction calculation', in
http://ireland.iol.ie/~geniet/eng/refract.htm#Calculated [accessed 2 July 2012].
Reijs, Victor M.M., and Kris Winer. 2016.'Calibration of magnetometer', github, in
https://github.com/kriswiner/MPU-9250/issues/42 [accessed 10 September 2016].
Schaefer, Brad E. 2000. 'New methods and techniques for historical astronomy and archaeoastronomy ',
Archaeoastronomy: The journal of astronomy in culture, Vol XV: pp. 121-35.
Smith, Andrew. 2014.'Horizon', in http://www.agksmith.net/horizon/ [accessed 10 September 2016].
Thom, Alexander. 1971. Megalithic lunar observatories (Oxford University Press).
Wilson, Michael. 2014.'¡¿ Mega-WHAT ?!: Megalithic horizon calendars, that's What', in http://mega-
what.com/index.html [accessed 16 December 2014].
Zawaski, M.J., and J.McKim Malville. 2007 to 2008. 'An astronomical survey of major Inca sites in Peru',
Archaeoastronomy: The journal of astronomy in culture, Vol XXI: pp. 20-38.

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