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digitally created using height terrain data remotely captured by airborne methods—photogrammetric or LiDAR. Since 2000, open-source space-based radar
height data, termed SRTM (Shuttle Radar Topographic Mission) by the National
Aeronautics and Space Administration of the United States (NASA) are available
with near- global coverage at a resolution on the ground of 90 m (c. three arc seconds) globally and 30 m (c. one arc second). The latter was released in 2015
(NASA Jet Propulsion Laboratory, 2014) and is suitable for generating horizon
profiles and panoramic views at user-specified locations (e.g., HeyWhatsThat
Panorama Viewer, 2019, and see Fig. 1b). ‘Horizon’ is another GIS tool used by
archaeoastronomers investigating the alignment of built structures (Smith, 2020).
This is open-source software which generates horizon profiles and scans as shown
in Fig. 3 using Scragg passage tomb, Co. Roscommon in north-west Ireland as an
example. Figure 3a shows a north-west to north-east section of the horizon at
Scragg cut from a full horizon scan using NASA’s one arc second SRTM height
data. The paths of the sun at summer solstice and the moon at major standstill are
additionally shown. The scene is valid for 3000 BC July 18 in the Julian calendar
system, the date of summer solstice at that time (NASA Jet Propulsion Laboratory,
2015). Figure 3c illustrates a polar plot of the maximum horizon distance versus
azimuth surrounding Scragg tomb with Horizon’s default distance bands of
10 km–40 km.
If the cursor is placed inside the Horizon Distance Window of the ‘Horizon’
programme, the display will interactively show azimuth and maximum horizon distance in that direction—an example of distance category by orientation. On a cautionary note, unless high-resolution LiDAR data is used, such digital tools will not
give an accurate horizon scan where the range is Restricted (<500 m). This makes it
necessary to use site-based naked-eye observations in such cases as next described.
Horizon scans were recorded at the Irish passage tombs using the method
described by Fraser (1983: 371). The magnetic bearings of junction points where
landscape distances changed from Restricted to Intermediate or Distant were
observed with a hand-held compass (Silva Sight Master graduated to 1°). These
were later corrected to azimuths using the magnetic declination of date (NOAA
National Centers for Environmental Information, 2015). For analysis, the horizon
was next conceptually divided into 36 sectors of 10° and the observed visibility data
compiled with a spreadsheet. Table 1 explains the method with an example: columns one and two show hypothetical field data, columns three and four show the
generalised equivalent.
Figure 4 shows the percentage frequency of occurrence of the three horizon categories or groups (Distant, Intermediate and Restricted) obtained at 266 sites, compiled in Microsoft Excel using 9576 discrete values of azimuth variables in a 36
column × 266 row matrix.
The Two-Sample Kolmogorov-Smirnov test (also known as K-S2) at the 0.05
level of significance determines if each of the three horizon categorical groups come
from the same distribution (Table 2). The results show that the three horizon categories are from different distributions and the null hypothesis is rejected.
The North Sky and the Otherworld: Journeys of the Dead in the Neolithic Considered
digitally created using height terrain data remotely captured by airborne methods—photogrammetric or LiDAR. Since 2000, open-source space-based radar
height data, termed SRTM (Shuttle Radar Topographic Mission) by the National
Aeronautics and Space Administration of the United States (NASA) are available
with near- global coverage at a resolution on the ground of 90 m (c. three arc seconds) globally and 30 m (c. one arc second). The latter was released in 2015
(NASA Jet Propulsion Laboratory, 2014) and is suitable for generating horizon
profiles and panoramic views at user-specified locations (e.g., HeyWhatsThat
Panorama Viewer, 2019, and see Fig. 1b). ‘Horizon’ is another GIS tool used by
archaeoastronomers investigating the alignment of built structures (Smith, 2020).
This is open-source software which generates horizon profiles and scans as shown
in Fig. 3 using Scragg passage tomb, Co. Roscommon in north-west Ireland as an
example. Figure 3a shows a north-west to north-east section of the horizon at
Scragg cut from a full horizon scan using NASA’s one arc second SRTM height
data. The paths of the sun at summer solstice and the moon at major standstill are
additionally shown. The scene is valid for 3000 BC July 18 in the Julian calendar
system, the date of summer solstice at that time (NASA Jet Propulsion Laboratory,
2015). Figure 3c illustrates a polar plot of the maximum horizon distance versus
azimuth surrounding Scragg tomb with Horizon’s default distance bands of
10 km–40 km.
If the cursor is placed inside the Horizon Distance Window of the ‘Horizon’
programme, the display will interactively show azimuth and maximum horizon distance in that direction—an example of distance category by orientation. On a cautionary note, unless high-resolution LiDAR data is used, such digital tools will not
give an accurate horizon scan where the range is Restricted (<500 m). This makes it
necessary to use site-based naked-eye observations in such cases as next described.
Horizon scans were recorded at the Irish passage tombs using the method
described by Fraser (1983: 371). The magnetic bearings of junction points where
landscape distances changed from Restricted to Intermediate or Distant were
observed with a hand-held compass (Silva Sight Master graduated to 1°). These
were later corrected to azimuths using the magnetic declination of date (NOAA
National Centers for Environmental Information, 2015). For analysis, the horizon
was next conceptually divided into 36 sectors of 10° and the observed visibility data
compiled with a spreadsheet. Table 1 explains the method with an example: columns one and two show hypothetical field data, columns three and four show the
generalised equivalent.
Figure 4 shows the percentage frequency of occurrence of the three horizon categories or groups (Distant, Intermediate and Restricted) obtained at 266 sites, compiled in Microsoft Excel using 9576 discrete values of azimuth variables in a 36
column × 266 row matrix.
The Two-Sample Kolmogorov-Smirnov test (also known as K-S2) at the 0.05
level of significance determines if each of the three horizon categorical groups come
from the same distribution (Table 2). The results show that the three horizon categories are from different distributions and the null hypothesis is rejected.
The North Sky and the Otherworld: Journeys of the Dead in the Neolithic Considered