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Information about Degree Confluences |
Definition
A confluence is defined as a flowing together; a meeting place (often of rivers).
In our case a degree confluence is the exact spot where an integer degree
of latitude and an integer degree of longitude meet, such as
43°00'00"N 72°00'00"W. The project
uses the WGS84 datum to define the confluence location.
Information about a Confluence
Initially, the information in our database about confluences was entered both manually,
and by extracting data from various sources.
Since then, there have been various updates, both manual and automated, and when someone
finds an error and lets us know, we make the correction.
Some confluences have been added or changed, where they were in the oceans
but a mathematical model shows that they should have a view of land.
The altitude listed for a confluence usually comes from a digital elevation model, and may
not be accurate.
The data used to make the distinction between 'Water' and 'Ice Cap' may not be accurate, and
polar ice caps change over time.
If you know of some confluence data that needs correcting, please advise the regional
coordinator for the area.
We have two sets of confluence information in our database. For all of the 64,442 possible degree confluences, we have some basic information. For the 24,654 degree confluences that we have "indexed", we have additional information, such as the nearest town, and we have assigned the confluence to a 'geographic area', such as a Country.
Land, Water, Ice
All of the 64,442 possible confluences have been assigned to one of three categories -
Land(21,544), Water(38,408),
or Ice Cap(4,490).
Confluences that are on 'Land' are those that our data shows as being located on
a continent or an island.
The presence of ice doesn't change that. A confluence on a lake doesn't change
it's status if the lake freezes over in the winter. A confluence located on a glacier
is considered to be on land.
Confluences not on 'Land' are on 'Water', except for areas with permanent ice, which
are categorized as 'Ice Cap', such as confluences near the North Pole.
Primary vs Secondary
A confluence is primary if:
Changes in the Confluence type
There are 39,788 confluences that are not "indexed"
in our database, because they lie in the oceans, are presumed to not have a view of land
(are more than 5 kilometers from land),
and are unlikely to be visited. An unindexed confluence is presumed to be Secondary,
and will be added to our database if visited.
A Primary confluence would be changed to Secondary if a visit shows that there is no view of land on a clear day. This would include a visit to a confluence on a large lake. A confluence that is Secondary because it is "on the water" and is presumed to have no view of land would be changed to Primary if a visit shows a view of land, but only if it qualifies based on the Poles Problem logic.
Confluences on borders
Confluences exactly on the Northern and Eastern borders of a state, province or country
will be included in that state, province or country. Confluences on the Southern and
Western borders will be included in the bordering state, province or country.
Historically, borders were defined by a variety of methods, and most that refer to specific coordinates were done long before the WGS84 system was defined. As a result, sometimes the above rule isn't needed for "border confluences". For example, the Canada/United States border, from 123°W on the the west coast to 96°W between Manitoba and Minnesota, when viewed using WGS84 coordinates, is actually a line that zig-zags back and forth across the 49th parallel. None of the confluences along the border are actually on the border, so they belong to the province/state in which they are located. If this part of the Canada/United States border was defined as the WGS84 49th parallel, then the above rule would apply, and all these confluences would belong to the United States. See the Canada page for more information.
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If the Earth were a perfect sphere, the north-south distance between adjacent pairs of degrees of latitude (parallels; lines that run east-west) would be the same from the equator to the poles. However, the east-west distance between adjacent pairs of degrees of longitude (meridians; lines that run north-south) varies depending on the latitude, with the maximum distance being at the equator, and the minimum distance being at the poles, where the lines of longitude meet. Because of the closeness of the degrees of longitude lines towards the poles, we designed a scheme that deals with this skewing of the collection of degree confluences towards the poles. The scheme designates every degree confluence as either Primary or Secondary. While Secondary confluences will be accepted if visited, they are not part of the primary goals of the Degree Confluence Project. The Earth is not a perfect sphere, and the WGS84 system that we use for degree confluences includes a mathematical model (GRS80) of the Earth as an ellipsoid. Using established GRS80 constants, and the Vincenty Algorithm (PDF document), the distance between degrees of latitude (lines that run east-west) varies from 110.57km (68.71mi) at the equator (0 degrees latitude) to 111.69km (69.40mi) between 89 degrees latitude and the poles. For the purposes of the project, we don't take these variations in the distance between degrees of latitude into account when categorizing degree confluences. Using the same calculation methods, the distance between degrees of longitude (lines that run north-south) varies between 111.32km (69.17mi) at the equator (0 degrees latitude) to 1.95km (1.21mi) at 89 degrees latitude, one degree from the north or south pole. Because the lines of longitude meet at the poles, the distance between degrees of longitude at the poles is zero. Starting at the equator, all degree confluences are designated as Primary, until the distance between degrees of longitude drops below 2/3 of the distance at the equator. At that point, two thirds of the confluences will be designated as Primary, so every third confluence will be designated as Secondary. This begins at 49 degrees. 111.32km (69.17mi) * 2/3 = 74.21km (46.11mi) The next change occurs when the distance between degrees of longitude drops below 2/3 of 2/3 of the distance at the equator. At that point, half the confluences will be designated as Primary. This begins at 64 degrees. 111.32km (69.17mi) * 2/3 * 2/3 = 49.48km (30.69mi) This scheme continues, using the "2/3 rule", until at 89 degrees, only ten of the possible 360 confluences will be considered as Primary, and at 90 degrees, there is only one confluence, and it is a Primary. The latitude ranges, and the fraction of Primary confluences in each range are: 0-48=1/1, 49-63=2/3, 64-72=1/2, 73-78=1/3, 79-82=1/4, 83-84=1/6, 85-86=1/8, 87=1/13, 88=1/20, 89=1/36. If you're going to visit a confluence at greater than 48 degrees latitude, use the search page to find out whether the confluence is Primary or Secondary. You can use either Ed Williams' Great Circle Calculator or the FAI World Distance Calculator to get the surface distance between two points of latitude and longitude. |
