The coordinate system of 1995 (SK-95) was established by Decree of the Government of the Russian Federation of July 28, 2002 No. 586 “On the establishment of unified state coordinate systems”. Used in the implementation of geodetic and cartographic work, starting from July 1, 2002.

Before the completion of the transition to the use of the SC, the government of the Russian Federation decided to use single system geodetic coordinates of 1942, introduced by the Decree of the Council of Ministers of the USSR of 04/07/1996 No. 760.

The expediency of introducing SK-95 is to improve the accuracy, efficiency and economic efficiency solving problems of geodetic support that meets modern requirements economy, science and defense of the country. Obtained as a result of the joint adjustment of the coordinates of the points of the space state network(KGS), the Doppler geodetic network (DGS) and the astronomical geodetic network (AGS) for the 1995 epoch, the 1995 coordinate system is fixed by points of the state geodetic network.

SK-95 is strictly coordinated with the unified state geocentric coordinate system, which is called "Parameters of the Earth 1990." (PZ-90). SK-95 is installed under the condition that its axes are parallel to the spatial axes of SK PZ-90.

Reference ellipsoid is taken as reference surface in SK-95.

The accuracy of SK-95 is characterized by the following root-mean-square errors of the mutual position of points for each of the planned coordinates: 2-4 cm for adjacent AGS points, 30-80 cm at distances from 1 to 9 thousand km between points.

The accuracy of determining normal heights, depending on the method of their determination, is characterized by the following mean square errors:

· 6-10 cm on average across the country from the level of leveling networks of 1 and 2 classes;

· 20-30 cm from astronomical and geodetic determinations during the creation of the AGS.

The accuracy of determining the excess heights of the quasi-geoid by the astronomical gravimetric method is characterized by the following root mean square errors:

· from 6 to 9 cm at a distance of 10-20 km;

30-50 cm at a distance of 1000 km.

SK-95 is different from SK-42

1) increasing the accuracy of transmitting coordinates over a distance of more than 1000 km by 10-15 times and the accuracy of the relative position of adjacent points in the state geodetic network by an average of 2-3 times;

2) the same distance accuracy of the coordinate system for the entire territory of the Russian Federation;

3) the absence of regional deformations of the state geodetic network, reaching several meters in SK-42;

4) the possibility of creating a highly efficient system of geodetic support based on the use of global navigation satellite systems: Glonass, GPS, Navstar.

The development of the astronomical and geodetic network for the entire territory of the USSR was completed by the beginning of the 80s. By this time, it became obvious that the general adjustment of the AGS was performed without dividing it into series of triangulation of the 1st class and continuous networks of the 2nd class, since a separate adjustment led to significant deformations of the AGS.

In May 1991, the general adjustment of the AGS was completed. According to the results of the adjustment, following characteristics accuracy of AGS:

1) root mean square error of directions 0.7 seconds;

2) the root mean square error of the measured azimuth is 1.3 seconds;

3) relative root-mean-square error of measurement of basic sides 1/200000;

4) the mean square error of adjacent points is 2-4 cm;

5) root-mean-square error of transmitting the coordinates of the source point to points at the edges of the network for each coordinate of 1 m.

The adjusted network included:

· 164306 items of 1st and 2nd class;

· 3.6 thousand geodetic azimuths determined from astronomical observations;

· 2.8 thousand basic sides in 170-200 km.

Astronomical geodetic network Doppler and KGS.

The volume of astronomical and geodetic information processed during joint adjustment to establish SK-95 exceeds the volume of measurement information by an order of magnitude.

In 1999, the Federal Service for Geodesy and Cartography (FSGiK) of the SGS of a qualitatively new level based on satellite navigation systems: Glonass, GPS, Navstar. The new GHS includes geodetic constructions various classes Accuracy:

1) FAGS (fundamental)

2) High precision WGS

3) Satellite geodetic network class 1 (SGS 1)

4) Astronomical geodesic network and geodesic networks of condensation.

WGS-84 has now become international system navigation. All airports in the world, in accordance with the requirements of ICAO, define their aeronautical landmarks in WGS-84. Russia is no exception. Since 1999, orders have been issued on its use in the system of our civil aviation(The latest orders of the Ministry of Transport No. HA-165-r dated 20.05.02 “On the performance of work on geodetic survey of aeronautical landmarks of civil airfields and airways of Russia” and No. HA-21-r dated 04.02.03 “On the implementation of recommendations on preparing ... for flights in the precision area navigation system ... ”, see www.szrcai.ru), but there is still no clarity on the main thing - whether this information will become open (otherwise it loses its meaning), and this depends on completely other departments that are not inclined to openness. For comparison: the coordinates of the ends of the runway of the airfield with a resolution of 0.01 ”(0.3 m) are currently issued by Kazakhstan, Moldova and the countries of the former Baltic states; 0.1” (3 m) - Ukraine and the countries of Transcaucasia; and only Russia, Belarus and all middle Asia reveal these vital data for navigation with an accuracy of 0.1" (180 m).

We also have our own global coordinate system, an alternative to WGS-84, which is used in GLONASS. It is called PZ-90, developed by our military, and besides them, by and large, no one is interested, although it has been elevated to the rank of state.

Our state system coordinates - "The coordinate system of 1942", or SK-42, (as well as the recently replaced SK-95) differs in that, firstly, it is based on the Krasovsky ellipsoid, somewhat larger than the WGS- 84, and secondly, "our" ellipsoid is shifted (by about 150 m) and slightly turned relative to the general earth. This is because our geodetic network covered a sixth of the land even before the advent of any satellites. These differences lead to a GPS error on our maps of the order of 0.2 km. After taking into account the transition parameters (they are available in any Garmin "e), these errors are eliminated for navigation accuracy. But, alas, not for the geodesic: there are no exact unified coordinate connection parameters, and this is due to local mismatches within the state network. Surveyors have to for each individual the district itself to look for the parameters of transformation into the local system.

Hello!
Today I will tell you, %USERNAME%, about shoes and sealing wax, cabbage, coordinate kings, projections, geodetic systems and just a little bit about web mapping. Get comfortable.

As Arthur Clarke said, any is enough advanced technology indistinguishable from magic. So it is in web cartography - I think everyone has long been accustomed to using geographic maps, but not everyone can imagine how it all works.

Here, it would seem simple thing - geographical coordinates. Latitude and longitude, which could be simpler. But imagine that you find yourself on a desert island. The smartphone has sunk, and you have no other means of communication. It remains only to write a letter asking for help and, in the old fashioned way, throw it into the sea in a sealed bottle.

That's just bad luck - you absolutely do not know where your desert island, and without specifying the coordinates, no one will find you, even if they catch your letter. What to do? How to determine coordinates without GPS?

So, a little theory to start with. To compare coordinates to points on the surface of the sphere, it is necessary to set the origin - the fundamental plane for counting latitudes and the prime meridian for counting longitudes. For the Earth, the equatorial plane and the Greenwich meridian are usually used, respectively.

Latitude (usually denoted by φ) is the angle between the direction to a point from the center of the sphere and the fundamental plane. Longitude (usually denoted θ or λ) is the angle between the plane of the meridian passing through the point and the plane of the prime meridian.

How to determine your latitude, i.e. the angle between the plane of the earth's equator and the point where you are?

Let's look at the same drawing from a different angle, projecting it onto the plane of our meridian. Let's also add a horizon plane to the drawing (a tangent plane to our point):

We see that the desired angle between the direction to the point and the plane of the equator equal to the angle between the horizon plane and the Earth's axis of rotation.

So how do we find this corner? Let's remember the beautiful pictures of the starry sky with a long exposure:

This point in the center of all the circles described by the stars is the pole of the world. By measuring its height above the horizon, we get the latitude of the observation point.

The question remains how to find the pole of the world at starry sky. If you are in the Northern Hemisphere, then everything is quite simple:

Find a bucket Ursa Major;
- mentally draw a straight line through the two extreme stars of the bucket - Dubhe and Merak;
- this straight line will point you to the handle of the Ursa Minor bucket. The extreme star of this pen - Polaris - almost exactly coincides with the North Pole of the world.

The polar star is always in the north, and its height above the horizon is equal to the latitude of the observation point. If you manage to get on North Pole, The North Star will be exactly over your head.

AT southern hemisphere It is not that simple. There is no peace near the south pole big stars, and you will have to find the constellation Southern Cross, mentally extend down its large crossbar and count 4.5 of its length - somewhere in this area will be located South Pole peace.

The constellation itself is easy to find - you have seen it many times on flags different countries- Australia, New Zealand and Brazil, for example.

Decided on the latitude. Let's move on to debt. How to determine longitude on a desert island?

Actually, this is very difficult problem, because, unlike latitude, the reference point of longitude (zero meridian) is chosen arbitrarily and is not tied to any observable landmarks. The Spanish king Philip II in 1567 appointed a substantial reward to anyone who would propose a method for determining longitude; in 1598, under Philip III, it grew to 6 thousand ducats at a time and 2 thousand ducats of annuity for life - a very decent amount at that time. The problem of determining longitude has been a fixed idea of ​​mathematicians for several decades, like Fermat's Theorem in the 20th century.

As a result, longitude began to be determined using this device:

In fact, this device remains the most in a reliable way determination of longitude (not counting GPS / Glonass) even today. This instrument… (drum roll)… marine chronometer.

In fact, when longitude changes, the time zone changes. By the difference between local time and Greenwich Mean Time, it is easy to determine your own longitude, and very accurately. Each minute of the time difference corresponds to 15 arc minutes of longitude.

Accordingly, if you have a clock set to Greenwich Mean Time (in fact, it doesn’t matter which one - it’s enough to know the time zone of the place where your clock is running) - don’t rush to translate them. Wait for local noon and the time difference will tell you the longitude of your island. (Determining the moment of noon is very easy - watch the shadows. In the first half of the day, the shadows are shortened, in the second, they are lengthened. The moment when the shadows began to lengthen is astronomical noon in the area.)

Both methods of determining coordinates, by the way, are well described in Jules Verne's novel "The Mysterious Island".

Geoid coordinates

So, we were able to determine our latitude and longitude with an error of several degrees, i.e. a couple of hundred kilometers. For a note in a bottle, such accuracy, perhaps, is still enough, but for geographical maps not anymore.

Part of this error is due to the imperfection of the tools used, but there are other sources of error. The Earth can be considered a ball only in the first approximation - in general, the Earth is not a ball at all, but a geoid - a body that most of all resembles a highly uneven ellipsoid of revolution. In order to accurately assign each point earth's surface coordinates need rules - how to project a specific point on the geoid onto a sphere.

Such a set of rules must be universal for all geographical maps in the world - otherwise the same coordinates will be in different systems designate different points earth's surface. At the moment, almost all geographic services use a single system for assigning a coordinate point - WGS 84 (WGS = World Geodetic System, 84 - the year the standard was adopted).

WGS 84 defines the so-called. reference ellipsoid - a surface to which coordinates are given for the convenience of calculations. The parameters of this ellipsoid are as follows:

Semi-major axis (equatorial radius): a = 6378137 meters;
- compression: f = 1 / 298.257223563.

From the equatorial radius and compression, you can get the polar radius, it is also a minor semi-axis (b = a * (1 - f) ≈ 6356752 meters).

Any point on the earth's surface, therefore, is associated with three coordinates: longitude and latitude (on the reference ellipsoid) and height above its surface. In 2004, WGS 84 was supplemented by the Earth Gravitational Model (EGM96) standard, which specifies the sea level from which heights are measured.

Interestingly, the zero meridian in WGS 84 is not at all Greenwich (passing through the axis of the passage instrument of the Greenwich Observatory), but the so-called. IERS Reference Meridian, which passes 5.31 arc seconds east of Greenwich.

flat maps

Suppose we have learned to determine our coordinates. Now you need to learn how to display the accumulated geographical knowledge monitor screen. Yes, that's bad luck - somehow there are not very many spherical monitors in the world (not to mention monitors in the form of a geoid). We need to somehow display the map on a plane - project it.

One of the most simple ways- project a sphere onto a cylinder, and then unfold this cylinder onto a plane. Such projections are called cylindrical, their characteristic property- all meridians are displayed on the map as vertical lines.

There are many projections of a sphere onto a cylinder. The most famous of the cylindrical projections is the Mercator projection (named after the Flemish cartographer and geographer Gerard Kremer, who widely used it in his maps, better known by the Latinized surname Mercator).

Mathematically, it is expressed as follows (for a sphere):

X = R λ;
y = R ln(tg(π/4 + φ/2), where R is the radius of the sphere, λ is the longitude in radians, φ is the latitude in radians.

At the output we get the usual Cartesian coordinates in meters.

The map in the Mercator projection looks like this:

It is easy to see that the Mercator projection distorts the shapes and areas of objects very significantly. For example, Greenland on the map takes twice large area than Australia - although in reality Australia is 3.5 times the size of Greenland.

Why is this projection so good that it has become so popular despite significant distortions? The fact is that the Mercator projection has an important characteristic property: it preserves angles when projected.

Let's say we want to sail from canary islands to the Bahamas. Let's draw a straight line on the map connecting the points of departure and arrival.

Since all meridians in cylindrical projections are parallel, and the Mercator projection also preserves angles, our line will cross all meridians at the same angle. And this means that it will be very simple for us to sail along this line: it is enough to keep the same angle between the ship’s course and the direction to polar star(or the direction to magnetic north, which is less accurate), and the desired angle can be easily measured with a banal protractor.

Similar lines crossing all meridians and parallels at the same angle are called loxodromes. All loxodromes in the Mercator projection are depicted as straight lines on the map, and it is this remarkable property, extremely convenient for marine navigation, that has brought the Mercator projection wide popularity among sailors.

It should be noted that what has been said is not entirely true: if we are projecting a sphere, but moving along a geoid, then the track angle will not be determined quite correctly and we will sail not quite there. (The discrepancy can be quite noticeable - after all, the equatorial and polar radii of the Earth differ by more than 20 kilometers.) An ellipsoid can also be projected with conservation of angles, although the formulas for the elliptical Mercator projection are much more complicated than for the spherical ( inverse transformation not expressed at all elementary functions). Complete and detailed description the mathematics of the Mercator projection on an ellipsoid can be found.

When we started making our maps at Yandex, it seemed logical to us to use the elliptical Mercator projection. Unfortunately, many other web mapping services don't feel this way and use spherical projection. So long time it was impossible to show tiles over the Yandex map, say, OSM - they diverged along the y axis, the closer to the pole - the more noticeable. In API version 2.0, we decided not to swim against the current, and provided the ability to both work with the map in an arbitrary projection, and show several layers on the map at the same time in different projections - whichever is more convenient.

Geodetic problems

Traveling on the loxodrome is very simple, but this simplicity comes at a price: the loxodrome will send you on a journey along a suboptimal route. In particular, the path along the parallel (if it is not the equator) is not the shortest!

In order to find the shortest path on the sphere, one must draw a circle centered at the center of the sphere passing through these two points (or, which is the same, intersect the sphere with a plane passing through two points and the center of the sphere).

It is impossible to project a sphere onto a plane in such a way that the shortest paths turn into straight segments; the Mercator projection, of course, is no exception, and the great circles in it look like strongly distorted arcs. Some paths (through the pole) in the Mercator projection cannot be correctly depicted:

This is how the shortest route from Anadyr to Cardiff is projected: first we fly to infinity due north, and then we return from infinity due south.

In the case of movement along a sphere, the shortest paths are built quite simply using the apparatus of spherical trigonometry, but in the case of an ellipsoid, the task becomes much more complicated - the shortest paths are not expressed in elementary functions.

(I note that this problem, of course, is not solved by choosing the spherical Mercator projection - the construction shortcuts is carried out on the reference ellipsoid WGS 84 and does not depend on the projection parameters in any way.)

During the development of the Yandex.Maps API version 2.0, we faced a difficult task - to parametrize the construction of the shortest paths so that:
- it was easy to use the built-in functions to calculate the shortest paths on the WGS 84 ellipsoid;
- it was easy to set your own coordinate system with own methods calculation of the shortest paths.

After all, the Maps API can be used not only to display maps of the earth's surface, but also, say, the surface of the Moon or some game world.

To construct the shortest paths (geodesic lines) in general case The following simple and unpretentious equation is used:

Here - the so-called. Christoffel symbols expressed in terms of partial derivatives of the fundamental metric tensor.

Forcing the user to parametrize his mapping area in this way seemed somewhat inhumane to us :).

Therefore, we decided to take a different path, closer to the Earth and the needs of our users. In geodesy, the problems of constructing the shortest paths are the so-called. the first (direct) and second (inverse) geodesic problems.

Direct geodetic problem: given starting point, the direction of travel (usually the course angle, i.e. the angle between north and heading), and the distance travelled. It is required to find the end point and the final direction of movement.

Inverse geodetic problem: given two points. It is required to find the distance between them and the direction of movement.

Note that the direction of travel (track angle) is continuous function, which changes along the way.

Having at our disposal the functions for solving these problems, we can use them to solve the cases we need in the Maps API: calculating distances, displaying the shortest paths, and constructing circles on the earth's surface.

We have declared the following interface for custom coordinate systems:

SolveDirectProblem(startPoint, direction, distance) - Solve the so-called first (direct) geodesic problem: where will we end up if we leave the specified point in the specified direction and pass the specified distance without turning.

SolveInverseProblem(startPoint, endPoint, reverseDirection) - Solve the so-called second (inverse) geodesic problem: build the shortest route between two points on the mapped surface and determine the distance and direction of movement.

GetDistance(point1, point2) - returns the shortest (along a geodesic) distance between two given points(in meters).

(The getDistance function is separate for cases where calculating distances can be done much faster than solving the inverse problem.)

This interface seemed to us quite simple to implement in cases where the user maps some non-standard surface or uses non-standard coordinates. For our part, we wrote two standard implementations - for the usual Cartesian plane and for the WGS 84 reference ellipsoid. For the second implementation, we used the Vincenty formulas. By the way, I directly implemented this logic, we say hello to him :).

All these geodetic features are available in the Yandex.Maps API starting from version 2.0.13. Welcome!

Tags:

• coordinates
• wgs84
• geodesy
• cartography

Add tags

Ellipsoid GRS80 (Geodetic Reference System - geodetic reference system) was adopted by the XVII General Assembly International Union geodesy and geophysics in Canberra, in December 1979 as a general earth reference ellipsoid.

The semi-minor axis of the GRS80 is parallel to the direction to the International Conventional Origin (EOR), and the prime meridian is parallel to the zero meridian of the BIE longitude count. The GRS80 is based on the theory of an equipotential (level or normal) ellipsoid. Ellipsoid GRS80 is recommended for carrying out geodetic works and calculating the characteristics of the gravitational field on the Earth's surface and in outer space.

Coordinate system pz-90.

The parameters of the Earth 1990 PZ-90 were determined by the Topographic Service of the Armed Forces of the Russian Federation. PZ-90 parameters include:

Fundamental astronomical and geodetic constants.

Characteristics of the coordinate base (parameters of the earth's ellipsoid, coordinates of points fixing the system, parameters of connection with other coordinate systems).

Models of normal and anomalous gravitational fields of the Earth, local characteristics gravitational field(height of the quasi-geoid above the global ellipsoid and gravity anomalies).

The coordinate system included in the PZ-90 is sometimes called SGS-90 (Satellite geocentric system 1990).

The beginning of the system is located at the center of mass of the Earth, the Z axis is directed towards the mean north pole for the middle epoch 1900-1905. (MUN). The X axis lies in the plane of the earth's equator of the epoch 1900-1905. and plane (ХОZ) defines the position of the zero-point of the accepted reference system of longitudes. The Y-axis completes the system to the right. Geodetic coordinates B, L, H refer to a common earth ellipsoid. The axis of rotation (semi-minor axis) coincides with the Z-axis, the plane prime meridian with a plane (XOZ).

The satellite geocentric coordinate system is fixed on the territory of the CIS with the coordinates of 30 reference points of the space geodetic network with an average distance of 1-3 thousand kilometers. For the PZ-90 system, the parameters of communication with the SK-42 and WGS-84 systems were obtained.

wgs-84 system.

The World Geodetic System WGS-84 (WorldGeodeticSystem-84) was developed by the US Department of Defense Military Mapping Agency. The WGS-84 system was implemented by modifying the NSWC-9Z-2 coordinate system, created from Doppler measurements, by bringing it into line with the data of the International Time Bureau.

The beginning of the WGS-84 system is located at the center of mass of the Earth, the Z-axis is directed to the Conditional Earth Pole (SZP), established by the BIE for the epoch 1980.0. The X axis is located at the intersection of the WGS-84 reference meridian and the USP equatorial plane. The reference meridian is the initial (zero) meridian determined by the BIE for the epoch 1980.0. The Y axis complements the system to the right, that is, at an angle of 90˚ to the east. The origin of the WGS-84 coordinate system and its axes also serve geometric center and axes of the WGS-84 reference ellipsoid. This ellipsoid is an ellipsoid of revolution. Its parameters are almost identical to those of the international GRS80 ellipsoid.

The WGS-84 system has been in use as the system for onboard GPS satellite ephemeris since January 23, 1987, replacing the WGS-72 system. Both systems were derived from Doppler measurements from the TRANSIT satellites. The carriers of the system were five stations of the GPS Control Segment. Since the mid-1990s, the network of WGS-84 stations has grown significantly. In 1994, the US DoD introduced an implementation of WGS-84 that was entirely based on GPS measurements. This new implementation known as WGS-84(G730), where the G stands for GPS and "730" stands for the week number (beginning at 0 h UTS January 2, 1994) when the National Display and Mapping Authority began presenting its GPS orbits on that system. The following implementations of this system:

WGS-84(G1150) for epoch 2001.0.

The WGS-84(G1150) reference frame is practically identical to the ITRF2000 reference frame.

Navigation is impossible without the use of coordinate systems. When using SNA for air navigation purposes, a geocentric coordinate system is used.

In 1994, ICAO recommended as a standard for all ICAO member states to use the WGS-84 global geodetic coordinate system from January 1, 1998, because in this coordinate system, the position of the aircraft is determined when using GPS systems. The reason for this is that the use of local geodetic coordinates on the territory of various states, and there are more than 200 such coordinate systems, would lead to additional error in determining the MVS due to the fact that the waypoints entered into the SNS receiver-indicator belong to a coordinate system that differs from WGS-84.

Centre global system coordinates WGS-84 coincides with the Earth's center of mass. The Z-axis corresponds to the direction of the normal earth's pole, which moves due to the earth's oscillatory rotation. The X-axis lies in the plane of the equator at the intersection with the plane of the zero (Greenwich) meridian. The Y-axis lies in the equatorial plane and is 90° away from the X-axis, the definition of the WGS-84 coordinate system is shown in Figure 4.

Figure 4. Definition of the WGS-84 coordinate system

AT Russian Federation, in order to provide geodetic support for orbital flights and solve navigation problems when using GLONASS, the geocentric coordinate system "Parameters of the Earth 1990" is used. (PZ-90). For the implementation of geodetic and cartographic work, starting from May 1, 2002, the system of geodetic coordinates of 1995 (SK-95) is used. The transition from the geodetic coordinate system of 1942 (SK-42) to SK-95 will take certain interval time before all navigation points on the territory of Russia will be transferred to new system coordinates.

The main parameters of the coordinate systems discussed above are presented in Table 5.

Coordinate systems used in navigation - Table 5

Parameter	Designation
Major axis, m
Minor axis, m
Offset from
center of mass
Earth on the axis, m
Orientation
relatively
axes, angles. sec.

The values ​​of ?x, ?y, ?z and ?x, ?y, ?z for PZ-90 are given relative to WGS-84, and for SK-95 and SK-42 relative to PZ-90.

Table 5 shows that the WGS-84 and PZ-90 coordinate systems are practically the same. It follows from this that when flying along the route and in the area of ​​the aerodrome, with the existing accuracy of determining the MVS, it does not matter in what coordinate system the navigation points will be determined.

The X-axis in WGS-84 and the X-axis in PZ-90 are the same.

The angular displacement of the Y axis "PZ-90 relative to the Y axis WGS-84 of 0.35" leads to a linear displacement on the surface of the ellipsoid at the equator of 10.8 m, and the displacement of the Z axis "with respect to the Z axis of 0.11" - 3.4 m. These displacements can lead to a general (radial) displacement of a point located on the surface of PZ-90 relative to WGS-84 by 11.3 m.

In order to be able to competently use any GPS receiver, you need to know some of its features. Let's talk a little about the shape of the Earth. We will need this in the future. Earth Shape, Datums. Many of us are used to representing our planet as a sphere. In reality, the shape of the Earth is a complex geometrically irregular figure. If we extend the surface of the waters of the World Ocean under all the continents, then such a surface will be called level. Its main property is that it is perpendicular to the force of gravity at any point. The figure formed by this surface is called the Geoid. For navigation purposes, the geoid shape is difficult to apply, so it was decided to bring it to a mathematical right body – ellipsoid of revolution or spheroid. The projected surface of the geoid onto the ellipsoid of revolution is referred to as Reference - Ellipsoesd. Since the distance from the center of the earth to its surface is not the same in different places, certain errors arise in linear distances. Each state, conducting geodetic and cartographic measurements, assigns its own set of parameters and orientation modes for the reference ellipsoid. Such parameters are called geodetic datums(Datum). The datum shifts (orients) the reference ellipsoid relative to a certain reference point (the center of mass of the Earth), setting more correct orientation relative to lines of latitude and longitude. Roughly speaking, this is a kind of coordinate grid tied to the reference ellipsoid of a particular place.

World Geodetic System 1984 (WGS–84) or World Geodetic System. Currently, the WGS84 system is controlled by an organization called the US National Geospatial-Intelligence Agency - NGA i.e. National Agency US geospatial intelligence. Initially, the WGS84 system was developed for air navigation purposes. March 3, 1989 Council international organization civil aviation ICAO, approved WGS84 standard (universal) geodetic reference system. The system entered the maritime transport industry after its adoption by the International Maritime Organization IMO.

At the heart of the orientation process WGS84 lies a three-dimensional system of geocentric coordinates. The reference point starts from the Earth's center of mass. The X axis lies in the plane of the equator and is directed to the meridian accepted by the International Bureau of Time (BIH). The Z axis is directed to the North Pole and coincides with the Earth's axis of rotation. The Y-axis completes the system to the right-hand one (rule right hand) and lies in the plane of the equator between the X-axis at an angle of 90° to the east.

The main parameters of the reference ellipsoid WGS84 include:

It should be remembered that the UKHO (United Kingdom Hydrographic Office) publishes its maps using about a hundred different datums (reference ellipsoids). But the GPS receiver determines the coordinates by default in the WGS84 datum. Looking ahead, most modern GPS receivers have the function of manual (manual) switching of the datum (i.e., the receiver’s memory contains great amount various datums). When transferring coordinates from the receiver to the map, it is necessary to check in advance in which Datum the map was published. To simplify this procedure, since 1982, the UKHO (United Kingdom Hydrographic Office) has added a note to the legend of their charts called “ position" and " Satellite Derived Position". In these paragraphs, we are informed about the Datum in which the map was published. And if it's not WGS84 - how to recalculate the coordinates. Pay special attention to this!