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Comparing programs that calculate the skyline profile

Computer programmes

There are, to my knowledge, three programmes, that are based on Shuttle Radar Topography Mission 90m (SRTM 3") and 30m (SRTM 1") resolution datasets. And these programmes calculate the skyline profile as seen from a certain location:

Panorama photos

Beside the above skyline profiles programmes, Mike Wilson manages the Mega-What website which has skyline profiles from several Irish Axial Stone Circles (ASC) based on real panorama photos and theodolite measurements. A great database!

So the results of the above three computer programmes have been compared with Mega-What theodolite measurements and panorama photos.

Comparison

The comparing results of the three programmes and theodolite measurements of the panorama photo can be seen in the below Fig. 1.

Comparing skyline profiles from
          HeyWhatsThat, SyntH and Horizon
Fig. 1  Comparing skyline profiles from
computer programmes: HeyWhatsThat (red
[SRTM 3"]), SyntH (darkblue [SRTM 1"]) and Horizon (lightblue [SRTM 3"] and yellow [SRTM 1"])
with theodolite measurements from Mega-What (black triangles)

SyntH (based on the 30m SRTM 1") is quite close to the theodolite measurements, while HeyWhatsThat (based on the 90m SRTM 3") a little less.
The computer skyline profiles don't map the Mega-What theodolite measurements sometimes, like: the small evaluation around 310 azimuth (this is a small nearby hill); and the peaks on the right side of the hill.
The skyline profile of Horizon programme seems to be somewhat out (see apparent altitudes around azimuth between 120 and 270 deg). The author of the programme states that the difference is likely to be due to vertical parallax: the close skyline (200m to 600m) and the different heights of the observer (which are indeed different between Horizon and HWT). For other azimuths (where the distance is further away [outside the azimuth range of 120-270deg]) such a difference is not seen.

Below Fig. 2 includes the Mega-What panorama photo. In an earlier panorama of Mega-What the shift of the small hill at azimuth of ~40 degrees was due to misalignment when stitching the panorama photos (and was improved with the help of the above computer programmes).

Photo of casheelkeetlye
Fig. 2  Comparing the computer programmes with panorama photo from Mega-What.

Summary

The above comparison shows that the SRTM 1" dataset is better (due to a higher resolution) than the SRTM 3" dataset. Both HeyWhatsThat and SyntH follow more or less the skyline shown in the Mega-What panorama photo. The SRTM resolution (min. 30m) is too small to depict nearby features (say closer than 1.6 km) accurate enough (say better a forth of the Sun/Moon's diameter) in the skyline.

The reason why Horizon (0.12a) has a different skyline seen for this location might be due to vertical parallax (close skyline).

Careful stitching of panorama photos is essential to overcome photo distortions and rotation errors. That is why theodolite (reference) measurements or computer programmes comparisons  are important.

For skylines further than 5km, all programmes and the photo give similar results.

Of course a photo (or theodolite measurements) can be most of the time better than a computer programme. But for photos one needs to have the opportunity to visit the site at clear weather conditions, which is not always possible. So computer programmes can help solve issues with far away skylines, due to atmospheric conditions they are sometimes not visible on photos/theodolite.

The height plateaus in HeyWhatsThat and SyntH could perhaps be made more natural(?) by using a proper smoothing/interpolation method. Fig. 3 provides a demo of this for HeyWhatsThat data (where the skyline goes between the middle of each plateau and the middle of each vertical line). The green data points (for 56% of the 360 deg) of HeyWhatsThat represent a skyline further than 1.6 km (red is nearer by).

Smoothed height plateaus
Fig. 3  Smoothing of the height plateaus

A combination of all methods (visits, photos, theodolite and computer programmes) is really important to appreciate reality.

References

Patat, Fernando. 2011. 'Horizon synthesis for archaeo-astronomical purposes', Astronomische Nachrichten, Vol 332: pp. 743-49.

Acknowledgments

I would like to thank the following people for their help and constructive feedback: Michael Kosowsky, Andrew Smith, Fernando Patat, Mike Wilson, and all other unmentioned people. Any remaining errors in methodology or results are my responsibility of course!!! If you want to provide constructive feedback, let me know.

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Major content related changes: May 24, 2015