The coordinate system is the key and difficult point of GIS software learning, and the use and processing of geographic data cannot be separated from the coordinate system. However, most of the colleagues with many years of work experience still have many problems with the understanding and use of the coordinate system, except for the coordinate system itself. The content of is relatively extensive and not easy to understand, but at the same time most people lack a summary of themselves. Now I will summarize the coordinate system more comprehensively, and explain in detail the understanding, use and existing problems of the coordinate system.
Everyone knows the related concepts of the coordinate system. The natural surface of the earth is a complex and irregular curved surface. On such a curved surface, we cannot scientifically calculate and transform the point information of various features on the ground. To solve this problem, we need to find a regular model, and this model can be expressed by mathematical formulas, and use this model to simulate the shape of the earth.
1. Geoid
We can regard the overall shape of the earth as a body surrounded by sea water, that is, imagine a static sea water that extends towards the interior of the continent, and finally forms a closed body. We call the surface of the sea at rest as a level There are infinitely many levels, one of which coincides with the average sea level is called the geoid, and the shape surrounded by the geoid is called the geoid. It is generally believed that the earth can represent the shape of the entire earth. After long-term research, it has been found that the earth is very close to a spheroid with slightly flattened poles. The spheroid close to the shape of the earth is called the earth ellipsoid. Regular surface expressed by mathematical formula.
2. Ellipsoid Orientation (Orientation by Agreement) To
determine the orientation of the ellipsoid's rotation axis, whether it is local or geocentric, it should meet two parallel conditions: the
short axis of the ellipsoid is parallel to the earth's rotation axis; the meridian of the earth is parallel At the beginning of astronomy meridian. The meridian plane of the beginning of the earth refers to the meridian plane passing through the Greenwich Observatory.
3. Ellipsoid positioning (coordinate origin positioning)
determines the position of the center of the ellipsoid, which is divided into one-point positioning and multi-point positioning. One-point positioning is to make the normal direction of the ellipsoid coincide with the direction of the plumb line at the selected earth origin. The ellipsoid is tangent to the geoid; multi-point positioning uses many large locations on the basis of one point positioning, and uses the first square method to constrain the repositioning. At the origin of the earth, the ellipsoid is no longer tangent to the geoid, but it is used Within the range of the astronomical geodetic network, the ellipsoid and the geoid have the best fit.
Four, several common coordinate systems
(1) Geographical coordinate system The
geographic coordinate system belongs to the spherical coordinate system. According to the difference of the projection surface, it is mainly divided into the astronomical geographic coordinate system and the geodetic coordinate system. The following mainly introduces the geodetic coordinate system.
The geodetic coordinate system uses the geodetic longitude L and the geodetic latitude B to indicate the position of the ground point on the ellipsoid of the earth, which is established on the basis of the reference ellipsoid. The spatial position of a point on the ground is represented by latitude (B), longitude (L), and height (H). The original meridian is defined as the meridian passing through the Greenwich Observatory, that is, the 0 degree longitude, also called the starting meridian. L is the angle between the meridian of the ground point and the original meridian, and B is the angle between the normal of the ground point and the equatorial plane , The height of the earth H represents the distance from the ground point to the ellipsoid along the normal line of the ellipsoid. The latitude and longitude on the topographic map are generally expressed in geodetic coordinates.
(2) The
coordinate origin of the spatial rectangular coordinate system is at the center of the earth or the center of the reference ellipsoid, the Z axis points to the north pole of the reference ellipsoid, the X axis points to the intersection of the starting meridian plane and the equatorial plane, and the Y axis lies on the equatorial plane and the X axis. vertical. Pointing complies with the right-hand rule.
(3) The Gaussian rectangular coordinate system
adopts the Gauss-Krüger projection method, and the spherical coordinates are projected onto the plane according to certain rules. The basic scale of our country adopts Gaussian projection 3° zone and 6° zone, no matter it is 3° Zone or 6° zone, both use the projected equatorial line as the Y axis and the central meridian as the X axis. The resulting coordinate system is called the Gaussian plane rectangular coordinate system. The coordinate axis of the Gaussian plane rectangular coordinate system points to the plane The coordinate axes in the geometry point exactly opposite.
(1) The 6° sub-zone
starts from the prime meridian (starting meridian, 0° meridian), and is divided by the longitude difference of 6°. The world is divided into 60 zones, numbered 1-60, and China spans 11 zones (13 Brought to zone 23).
(2) The 3° zone
is divided from 1.5° east longitude, and the longitude difference is 3°. Globally, there are 120 zones, and China spans 22 zones (24 to 45 zones).
(3) The conversion relationship between the belt number and the central meridian.
Find the central meridian with a known belt number:
6° sub-band: L=6N-3 3° sub-band: L=3N
where L represents the central meridian and N represents the belt number.
Find the band number when the longitude is known:
6° band: N=int(L/6) 3° band: N=int[(L-1.5)/3]