Technical Note: Sightline Analysis

This article discussed technical aspects of the sightline analysis tool in Photo Ephemeris Web. You can read about how to access and use this tool here.

Overview

The sightline analysis tool queries for an elevation profile between a pair of coordinates (latitude, longitude), start and end.

A sightline falls along a geodesic path (great circle), rather than line of constant bearing (rhumb line or loxodrome). 150 elevation points are requested via a web service for points lying on the geodesic between start and end.

From the elevation data, we construct the sightline analysis by calculating the apparent altitude from the start position to each point along the path. The apparent altitude is calculated using an inverse geodetic calculation referenced to the WGS84 ellipsoid, and allowing for the effects of atmospheric refraction. This gives improved accuracy in higher latitudes where a spherical model would be less appropriate.

A point is deemed visible if the corresponding apparent altitude is greater than the maximum apparent altitude encountered along the path so far, progressing from start to end.

Man-made structures and Vegetation

The sightline analysis does not account for man-made structures or vegetation, except that the effect of any elevation offset applied to the map pins is included. Intervening buildings or trees along the sightline between these points are not shown.

Data Sources

The following data sources are used:

• 3DEP (formerly NED and NED Topobathy) in the United States, 10 meters outside of Alaska, 3 meter in select land and territorial water areas
• ArcticDEM strips of 5 meter mosaics across all of the land north of 60° latitude, including Alaska, Canada, Greenland, Iceland, Norway, Russia, and Sweden
• CDEM (Canadian Digital Elevation Model) in Canada, with variable spatial resolution (from 20-400 meters) depending on the latitude.
• data.gov.uk, 2 meters over most of the United Kingdom
• data.gv.at, 10 meters over Austria
• ETOPO1 for ocean bathymetry, 1 arc-minute resolution globally
• EUDEM in most of Europe at 30 meter resolution, including Albania, Austria, Belgium, Bosnia and Herzegovina, Bulgaria, Croatia, Cyprus, Czechia, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Kosovo, Latvia, Liechtenstein, Lithuania, Luxembourg, Macedonia, Malta, Montenegro, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, and United Kingdom
• Geoscience Australia's DEM of Australia, 5 meters around coastal regions in South Australia, Victoria, and Northern Territory
• GMTED globally, coarser resolutions at 7.5", 15", and 30" in land areas
• INEGI's continental relief in Mexico
• Kartverket's Digital Terrain Model, 10 meters over Norway
• LINZ, 8 meters over New Zealand
• SRTM globally except high latitudes, 30 meters (90 meters nominal quality) in land areas

The following map gives an overview of which data sources are used by location:

The maximum horizontal (ground) resolution stated above may be higher or lower than the maximum resolution available from the web service, which is limited to 4.8m at the equator (3.4m at 45°, 2.4m at 60°).

Accuracy

Each digital elevation model has its own accuracy limits, but for the SRTM model, the vertical accuracy is given as ±16m at 95% confidence (i.e. 95% of the time, the elevation value will be  within ±16m of the true value). Other studies have indicated that it actually performs significantly better than this.

However, to allow for some error in the data sources, the sightline profile shows two states: the 'as calculated' sightline and an 'optimistic' sightline, which allows a 1% margin in the max apparent altitude calculation, i.e. if a the max apparent altitude of a point is within 99% of the current max, it is still considered 'visible'. These two treatments are overlaid in the sightline chart. The net effect is to show a small extension to visible path segments by virtue of the optimistic treatment:

The rationale for the inclusion for the optimistic treatment is that, in photography planning, avoiding a false negative (showing something as not visible, when in fact it is) is a worse outcome than a false positive (showing it as visible, when it's not). Users can choose to disregard the optimistic treatment and rely only on the 'as calculated' sightline. The right choice will depend on your circumstances and photographic priorities.

Attribution

• ArcticDEM terrain data DEM(s) were created from DigitalGlobe, Inc., imagery and
  funded under National Science Foundation awards 1043681, 1559691, and 1542736;
• Australia terrain data © Commonwealth of Australia (Geoscience Australia) 2017;
• Austria terrain data © offene Daten Österreichs – Digitales Geländemodell (DGM)
  Österreich;
• Canada terrain data contains information licensed under the Open Government
  Licence – Canada;
• Europe terrain data produced using Copernicus data and information funded by the
  European Union - EU-DEM layers;
• Global ETOPO1 terrain data U.S. National Oceanic and Atmospheric Administration
• Mexico terrain data source: INEGI, Continental relief, 2016;
• New Zealand terrain data Copyright 2011 Crown copyright (c) Land Information New
  Zealand and the New Zealand Government (All rights reserved);
• Norway terrain data © Kartverket;
• United Kingdom terrain data © Environment Agency copyright and/or database right
  2015. All rights reserved;
• United States 3DEP (formerly NED) and global GMTED2010 and SRTM terrain data
  courtesy of the U.S. Geological Survey.