- Sun and Moon, geostationary orbit
- Global positioning system (GPS)
- Geographic coordination system
- Geoid and ellipsoid, sea level
- Magnetic pole
- Map of the World
Observing the space we are in
Sun and Moon
If we observe outside of Earth, there are, between others, two Important objects around to be found:
- The first object is Sun. The Sun is in the distance of 150 million
kilometers from Earth. At the speed of light of 300,000 km/s (3×108 m/s) light beam travels 8 minutes between Sun and Earth. Earth rotates around the Sun at a period of 365 days.
- The second object is Moon. The Moon is in distance of over 300,000
km from Earth. That means light needs to travel little over 1 second. The period in which Moon rotates around Earth is significantly shorter. It is 28 days.
The arrangement of these 3 bodies anomaly forms unique stage:
- A lunar eclipse
The Earth is just between Moon and the Sun.For several minutes shade from the Earth can be observed when watching Moon. Such occurrence is very seldom and can occur only on full Moon.
- A solar eclipse
The Earth is just behind the Moon, therefore light from the Sun can not pass.Such occurrence is very seldom as well and can occur only on new Moon.
What causes the shade on the Moon, which we observe on daily basis?
Contrary to a popular belief, shade visible almost daily on Moon is caused not by Earth, but by Moon itself.
Geosynchronous and geostationary orbit
Global positioning system (GPS)
In 3D space, we need 3 distances from known points to locate our unknown position:
Today there are around 30 GPS satellites cruising. Their altitude 20,200 km is lower to altitude of the geostationary orbit. Therefore, they cruise faster (they run around Earth 2 times per a day), so they appear to move when being observed from Earth. We can typically track around 10 satellites every time and the number changes during the day.
Limitations of GPS
- The equipment may be not available.
- Sometimes conditions do not allow to use GPS at all. It may be the case of town/cities: measurements near buildings may be not possible, because constructions prevent satellites from being visible. Measurement can be conducted under roof. It may be also case that surveying is conducted in a forest or under ground.
- The precision of GPS might be not satisfactory for surveying
purpose. Handheld GPS can have precision of up to several metres. Advanced (expensive) GPS are improved to have precision of centimeters. Such precision still might be not as high as requested for example when leveling road surface.
GPS is in use for upgrading current networks now. However, because of factors written above, classic geodetic equipment are still basic need for surveying engineer.
Improved concept of GPS for geodetic surveying
Differential GPS positioning (DGPS)
There is a number of reasons behind systematic error. Only some of them are named here:
- Orbit errors ("winds", gravitational pulls, ...)
- Satellite geometry
- Atmospheric interference (humidity, temperature, pressure, ionized air)
Real time kinematics (RTK)
Having knowledge of speed of sound can become in some cases useful as well. The speed is 1,236 km/h or 343 m/s. In other words, in 1 second sound travels roughly 3 km and that is information worth remembering.
Geographic coordination system
Radius of Earth is 6,378 km. It can be computed (2πr) that circumference (eg. length of the equator) is around 40,000 km.
- Longitudes appear vertical and are of the same curvature. Zero degrees longitude (prime meridian) passes Royal observatory in Greenwich, England.
- Latitudes differ in length and are parallel to
each other. Zero degrees latitude is equator.
Tropic of cancer (Northern tropic) is latitude of 23.5° North, Tropic of Capricorn is 23.5° South. Tropics enclose location where the Sun "travels" in the period of a year.
We use a degree of longitude (West or East), latitude (North or South) together with altitude to describe locations on Earth.
Geoid and ellipsoid
For many purposes Earth can be simplified to be viewed as a sphere. Two terms are connected with this topic: Geoid and ellipsoid:
- Geoid could be seen if the whole surface is covered by water and surface is affected only by gravity of Earth.
- Ellipsoid is a mathematical attempt to describe Geoid in simple form.
The Geoid is used to describe heights. Ocean's water level is registered at coastal places over several years.
Every nation or group of nations have established their mean sea level points.
The Geoid is used to describe heights. In order to establish the Geoid as reference for heights, the ocean's water level is registered at coastal places over several years. The resulting water level represents an approximation to the Geoid and is called the mean sea level.
Every nation or group of nations have established those mean sea level points, which are normally located close to the area of concern. For the Netherlands and Germany, the local mean sea level is realized through the Amsterdam tide-gauge (zero height). From that sea level height of other points can be found by geodetic levelling. The sea level at particular area is affected by ocean currents and climate.
Obviously, there are several realizations of local mean sea levels (also called local vertical datums) in the world. They are parallel to the Geoid but for other reasons offset by up to a couple of meters.
Care must be taken when using heights from another local vertical datum. This might be the case in the border area of adjacent nations. An example, the tide gauge (zero height) of the Netherlands differs -2.34 metres from the tide gauge (zero height) of the neighbouring country Belgium (figure below). Even within a country, heights may differ depending on to which tide gauge, mean sea level point, they are related. An example, the mean sea level from the Atlantic to the Pacific coast of the USA increases by 0.6 to 0.7 m.
GPS and heights
- The most convenient geometric reference is the oblate ellipsoid
- For small scale mapping purposes a sphere may be used (the global horizontal datum)
- For local areas local ellipsoid is being used (the local horizontal datum)
Above was defined a physical surface as a reference to measure heights. Since we project horizontal coordinates onto a mapping plane, the reference surface for horizontal coordinates requires a mathematical definition and description. The most convenient geometric reference is the oblate ellipsoid, though for small scale mapping purposes a sphere may be used. Global horizontal datums, such as the ITRF2000 or WGS84, are also called geocentric datums because they are geocentrically positioned with respect to the centre of mass of the Earth.
Local and global ellipsoids
Local ellipsoids have been established to fit the Geoid (mean sea level) well over an area of local interest, which in the past was never larger than a continent. This meant that the differences between the Geoid and the reference ellipsoid could effectively be ignored, allowing accurate maps to be drawn in area near the datum (figure below).
We are used to use a magnetic compass to find directions. However using a compass is a bit tricky:
- Magnetic compass points to magnetic north, not to geographic north. This can be not much an issue for common use until you are not close to the pole.
- Magnetic north is not within stable location but travels through time.
- Using compass can be trickier in some locations with magnetic
noise, since Earth is not the only source of magnetic field.
- Limited precision of reading.
Map of the World
It is expected that who has an university degree, has also some general knowledge. Anyone who has passed university has to be confident to find at least all G-20 members on the map and some other important locations as well.
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