Coordinate System Definition in GIS

In previous posts I have discussed coordinate transformation & the basics of coordinate systems, following on from this, I thought it would be useful to share some information about coordinate system parameters & the UK standard which I mostly work to.

Coordinate systems 101 – Parameters

If you work with geographic data then you will have come across all the information that relates to a coordinate system or datum, but what does it mean? No doubt, you will have come across something like this?

Coordinate Parameters

First of all note that there are 2 headers here, HORIZONTAL and VERTICAL coordinate reference system. The horizontal relates to your everyday x,y coordinate system whereas the vertical relates to the height [also known at Z or H]. Normally, on most maps or charts, the vertical coordinate system is not required as the end user only wishes to know the conversions and parameters that have been used to translate the real world spherical data into a flat map. This isn’t to say that the vertical reference shouldn’t be used, it is quite important when creating offshore mapping or maps with suggested heights so that objects like MHW [Mean High Water] can be interpreted correctly – MHW is affected by height and therefore will appear greater or larger depending on vertical reference.

Already we have answered part of the question originally asked as stated in the last paragraph – where the paper/printed/web map is a two dimensional representation of what would be curved or have height in the real world, the user needs to understand how this has been done. There are several reasons why, some users are civil engineers and need to ensure accuracy, so users are legal and have to meet standards, others are just really OCD and want to ensure that the latest and most up to date translations are being used. Let’s go through what each of the terms mean:

EPSG coordinate reference system code: Sometimes called the CRS, EPSG or SRS code – Each “approved” coordinate system has a 5 digit code which can be EPSG Registry Database and provides all the necessary information about a coordinate system. The official geomatics codes are held by the International Association of International Oil and Gas Producers.

EPSG dataset version: The version of the reference system which is being used, this is relevant for systems based on dynamic datum & to ensure the latest, most up to date version is being used.

Geographic coordinate reference system name:  This relates to the GEOGRAPHIC [datum] SYSTEM name which is used. Sometimes also called the GRS [Geographic Reference System] or the GCS [Geographic Coordinate System].

Ellipsoid name: Also called spheroid. The ellipsoid/spheroid refers to the reference surface being used to emulate the Earth. This will be specific to this area of the world (sometimes called a local datum or if best fitting the whole earth a global datum). The image below shows how the local datum is quite poorly shaped globally but for the specific area of need it is extremely accurate whereas the global datum is a “best fit” all around, not amazing accurate everywhere but the best compromise.

Local and global

Ellipsoid semi-major axis:  If we take the above image, the semi-major axis is the distance from where the equator touches the surface to the centre of the spheroid, the correct terminology is “The longest radius of an ellipse”, this is described [commonly] as a distance in metres.
Ellipsoid inverse flattening: For some reason, rather than provide the semi-minor axis, we [geographers] prefere to give the inverse flattening, this is simply a measure of how much the symmetry axis is compressed relative to the equatorial radius, commonly provided in metres.
Map Projection name: This is the local coordinate system name, for example “British National Grid” or “Zone 30N”. This should only ever be used with other information as there can be several projections which use the same name but use different GCS or parameters. For example, “UTM Zone 30N” exists with both WGS84 datum and with ED50 datum.
Map Projection method name: This refers to the method by which the “flat” is fitted to the “round”. There are several methods, with each preserving either area, shape, direction or scale. Because the sphere is not a developable surface, it is impossible to construct a map projection that is both equal-area and conformal. Map Projection Methods

projection_families

 

latitude of natural origin: Also called the “latitude of projection origin”, it is the latitude chosen as the origin of rectangular coordinates for a map projection.
longitude of natural origin: Also called the “longitude of projection origin”, it is the longitude chosen as the origin of rectangular coordinates for a map projection. The important thing to remember about the latitude and longitude of origin is that they don’t affect the distortion pattern of the map. All they do is define where the map’s x,y units will originate.

False origin
scale factor at natural origin: This is a multiplier for reducing a distance obtained from a map by computation or scaling to the actual distance at the projection origin
false easting: The value added to all “x” values in the rectangular coordinates for a map projection. This value frequently is assigned to eliminate negative numbers.
false northing: The value added to all “y” values in the rectangular coordinates for a map projection. This value frequently is assigned to eliminate negative numbers.
Projected CRS axes units name: This relates to the units to be used for x,y ccordinates (Normally metres but don’t be surprised by feet & inches!)

How much of this should I put on a map?

This seems a little overkill doesn’t it? Well, not really, there have been several occasions where I have had to create my own coordinate system to more accurately map a local area, the only way to relate this is to provide all those parameters so that the GIS can properly relate it to the map.

Although it is commonly used in the offshore industry, there are guidelines for what should be displayed and provided which has been set out by the OGP [the holders of the EPSG registry]. You can read them here: Coordinate reference system definition —recommended practice

Nick D

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