preface

The author of this article is foreign cartographer Heather Smith, translated and published by Diandian GIS, if there are mistakes and omissions, please point out the background message

Coordinate system is the basic knowledge of GIS learners. But there’s so much jargon that’s hard to understand!

When working with coordinate systems, I compiled a list of things that used to confuse me. Hopefully these explanations will help you out as well.

What is the difference between GCS and PCS?

The Geographic Coordinate System (GCS) is a frame of reference that limits the location of features in the earth’s model. It’s shaped like a sphere — spherical. Its units are angles, usually degrees.

The projected coordinate system (PCS) is flat. It contains a GCS, but it uses mathematics (projection algorithms) and other parameters to convert that GCS to a plane. It has linear units, most commonly meters.

GCS is necessary for data to understand the exact position of the earth’s surface. PCS are required to draw data on a flat map.

For more information, consult the Geographic and projection coordinates in the ARCGIS help.

What is the difference between datum and geographic coordinates?

Datum is a parameter in the geographic coordinate system (GCS).

The datum is part of the GCS for determining which models (spheres) represent the earth’s surface and its position there with respect to the surface. Because the earth’s surface is not completely smooth or round, many different datum levels have been designed for different parts of the world.

A complete definition of how the GCS is matched to the coordinate values of the real position on earth. In addition to datum, the GCS includes prime meridians (specifying the location of 0° longitude) and units of Angle (usually degrees).

The datum plane consists of an ellipsoid defined by its major semi-axis, minor semi-axis, and antiflattening values.

What’s the difference between a projection and a projected coordinate system?

Projection is a parameter in the projection coordinate system (PCS).

The projection is defined by the mathematical algorithm on how to render a circular ground plane map. You can view all projections available in ArcGIS in the list of supported map projections (pro.arcgis.com/zh-cn/pro-a…

A PCS is a complete definition of how a particular circular earth model is projected onto a flat map. In addition to projections, PCS includes geographic coordinates (defining the earth model), units (usually meters), and a set of parameter values that vary depending on the projection (eastward shift, central meridian, standard parallel, etc.). These can be used to centralize PCS in different parts of the world.

As the name implies, PCS is a coordinate system. Projection is not a coordinate system; It is an algorithm used to create PCS.

What is the difference between WKT and WKID?

These are all ways to identify coordinate systems, so you can be sure that you are using exactly the same parameters as everyone else.

Well-known Text (WKT) is a string that defines all the necessary parameters of the coordinate system. Save the projection file (.prj) of any coordinate system and open it in a text editor to view its WKT.

The ID (WKID) is the unique number assigned to the coordinate system. You can find WKID in the coordinate system details window. Once you know this number, you can easily search the coordinate system later.

The WKID authority is EPSG (European Petroleum Survey Group) or Esri, but these numbers do not overlap, so there is no need to worry about which authority defines the ID.

What is the difference between defining a projection tool and a projection tool?

The projection definition tool is mainly used when data has an unknown coordinate system. It reallocates coordinate system information in metadata. It doesn’t change the actual coordinates.

The project’s tool transforms from one all coordinate system to another. It also updates metadata information.

For more information, see the ARCGIS Help for a definition projection or project?

What is the difference between dynamic projection and geographic transformation?

Geographic transformation is part of the dynamic projection process.

Dynamic projection is the method ArcGIS uses to resolve conflicts when your data is in a different coordinate system than the map. If you do not have an instant projection, you will not be able to plot the data until you have first converted it to the same coordinate system as the map.

When dynamic projection includes transformations between geographic coordinate systems, the process will include geographic transformations. These are calculations that convert coordinates from one GCS to another. They are sometimes referred to as benchmark transformations.

You can choose which transformations to use. If your data uses a different GCS than the map, and you do not use geographic transformations, the data will be plotted in the wrong location.

For more information, check out dynamic Projections and Geographic Transformations in ARCGIS Help.

What is the difference between a spatial reference and a coordinate system?

It doesn’t make any difference. At least not with Esri products. These terms are used interchangeably.

You can watch a video about coordinate systems and map projections (Youtu.be /PICwxT0fTHQ… .

Many thanks to Bojan š avrič and Melita Kennedy for their help and support in writing this and other recent articles!

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