Wednesday, January 8, 2020

What's EPSG And How Do I Use It?

Standardized and simplified spatial references


EPSG stands for European Petroleum Survey Group. They publish a database of coordinate system information. The EPSG number is a 4 to 6 digit number that represents the parameters of a coordinate reference system. EPSG numbers are embedded in georeferenced files, and assigned by the user to mapping projects that can display those files.

Huh?

There are projected coordinates, and geographic coordinates.

Geographic coordinates will have:
  • Projection (lat/long is implied)
  • Datum (WGS84, for example - datum defines the 0,0 coordinate for the projection)
  • Units (decimal degrees is implied)
  • Ellipsoid (WGS84, for example - ellipsoid defines the Earth-shaped model to be used)

Projected coordinates look like this:
  • Projection (UTM, for example)
  • Zone (Defines the longitude to center the map on, unique to the projection)
  • Datum (NAD83, for example - datum defines the 0,0 coordinate for the projection)
  • Units (Meters, for example)
  • Ellipsoid (WGS84, for example - ellipsoid defines the Earth-shaped model to be used)

GIS software and some CAD software will reproject spatial data on the fly where source materials and projects they link to have mismatched parameters based on reading the coordinate reference system, so much of the time you won't need to do anything. If your software can not reproject on the fly, you'll need to make sure all your data is using the same coordinate system manually.


Where Do I Find My EPSG Number?

In any software, find the properties for the data view and locate the spatial reference system information. It's often written like "NAD83 UTM zone 17N" but you will see the "EPSG Authority" information listed along with false easting and northing, central meridian, scale factors, transformation information etc. Coordinate system information for data layers is listed with the source information. The EPSG number may be listed explicitly, or, as shown below, described with well known text (WKT).

Data Frame Properties - NAD_1983_UTM_Zone_17N
WKID: 26917 Authority: EPSG

Layer Properties - Projected Coordinate System: NAD_1983_UTM_Zone_17N


Common EPSG Codes

See the EPSG projections below that are most often requested by our customers, with Canada on the left and a close up of Toronto on the right.

EPSG:4326 - Geographic (aka WGS84, Used in the Google Earth globe)
EPSG:3857 - Web Mercator (Most common system for projected online maps like Google Maps)
EPSG:3978 - NAD83 Canada Atlas Lambert (Best for nation wide coverage)
EPSG:32190 - NAD83 MTM 10 (Often used in Toronto)
EPSG:26917 - NAD83 UTM zone 17 (Often used for Southern Ontario)
EPSG:26912 - NAD83 UTM zone 12 (Often used for Alberta)

This is not an exhaustive list. For every combination of NAD27 or NAD83, UTM or MTM, Original or CSRS, etc there will be a unique EPSG number, although many of these coordinate systems will produce very similar looking maps. More EPSG numbers can be found here.

EPSG:4326 (Geographic)
EPSG:3857 (Web Mercator)
EPSG:3978 (NAD83 Canada Atlas Lambert)
EPSG:32190 (NAD83 MTM 10)
EPSG:26917 (NAD83 UTM zone 17)
EPSG:26912 (NAD83 UTM zone 12)


How Do I Update Or Change The EPSG Number?

If you need to ask the question, you probably don't have the tools to do it. Please contact First Base Solutions for data conversion services, or request delivery in your desired coordinate system when you place an order for data. A fee for custom labour may apply.


http://www.firstbasesolutions.com/



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