Geo-Referencing of GIS - facegis.com
Geo-Referencing

Registration and georeferencing

Registration:   lining up layers with others in a dataset

Geo-registration (georeferencing): linking a layer or dataset with spatial coordinates

If the world was flat, we could use a simple coordinate system. with 0.0 in the bottom left corner. But its NOT.. (DOH!! )

1. The GEOID, ellipsoids and North American Datum (NAD)

Ground coordinates are based on a surveyed ellipse that approximates the Earth. This ellipsoid has slightly different equatorial and polar radii, due to 'polar flattening'. Although it is only about 1/300th of the earth's radius, the calculation of this ellipsoid over the last century has given different ground coordinates. Georeferencing-Geog205

Today, there is agreement on the World Geodetic System 1984 WGS-84

Coordinates are based on a DATUM; "a basis for mapping"- a reference from which measurements are made ... there are many local datums - list

In North America there have been two standards (North American Datum = NAD)

NAD27 based on Clarke's ellipsoid (1866)
• measured by traditional survey; was used for conventional mapping

NAD83 based on GRS80 = WGS84

• measured by satellite geometry; is now our standard for digital mapping

There can be a difference (NAD27-NAD83) of up to 170 metres in y (N-S) and 70 metres in x (E-W), corresponding to the difference between the two ellipsoids. This is known as the 'NAD shift'. GIS software can make this conversion, otherwise data based on the two different NAD standards will not 'match' or overlay at large scales.

The adoption of a new ellipsoid has also changed the exact location of graticule lines: Prime Meridian marker Matt

2. The Graticule and Geographic (Unprojected) data

The Graticule

• Lines of longitude (meridians) - converge at Poles
• Lines of latitude (parallels) - are equally spaced
• Measures in degrees, subdivided into minutes and seconds (1 degree = 60 minutes, 1 minute = 60 seconds)

Problems in using the graticule in GIS:

• The graticule is not a 'rectangular' system and is hence poorly suited to GIS, since 'one degree', and also areas, are not uniform across the world. (for range, see GEOG205, lecture 3)
• The subdivisions are NOT decimal (minutes and seconds are 'sexagesimal'). In GIS, the best notation for 'geographic' digital data is as decimal degrees (e.g. 53 degree 30 minutes = 53.50).
• Since we have a '0' line both in latitude and longitude with values either side, latitude and longitude in GIS involves negative numbers south of the equator (latitude) and west of the Greenwich Meridian.

Although geographic data are sometimes referred to as 'unprojected', the format corresponds to the Plate Carree

Geographic is a suitable system for storing global data, but NOT for display or analysis

e.g. the Natural Resources Canada toporama website (PG map sheet is 93G or 93G15 (1:250,000 / 1:50,000)

3. Map projections - 'projected' data

A map projection is a method of showing the 3-D planet Earth on a 2-D plane, (inevitably causing distortions). The distortion can occur in distance, area, shape, or direction. Each projection will result in different types and amounts of distortion.

The 3 main types of projections are: shown in this graphic

Azimuthal (or planar) - all points maintain their true compass bearing from the centre.
Cylindrical - lines of longitude and are of equal length.
Conic - distortion increases away from the standard parallel (where the cone touches the globe.)

Generally, there is least (none) distortion where the globe touches the projection surface, but distortion increases away from this point or line(s)... and to further complicate matters, these can be modified by projection orientation

For more information on map projections, go to the Geography 205 lecture on Map Projections

The most common georeferencing system in Canada, used on our topographic maps is the Universal Transverse Mercator (UTM) system, based on the Transverse Mercator projection (below).

Mercator Projection (1569)

• Most common projection to 20th century (cylindrical projection)
• Lines of constant compass bearing (called "rhumb lines")
• Maintains shapes (N-S is stretched equally to E-W)
• Low distortion near equator
• Areas enlarged away from equator (cannot show poles)

Transverse Mercator

• 'Flipped' 90 degrees from standard Mercator
• Low distortion near central meridian
• Areas enlarged away from CM
• Basis for the UTM system

The UTM system is used for mapping and GIS in many countries.

• Adjacent map sheets and digital tiles match within a zone
• Adjacent sheets and tiles DON'T match between zones
• The UTM grid lines up with the graticule in the centre, but NOT elsewhere
• Used for scales > = 1 : 250,000; not used for smaller scales
• Adopted after world war II (after 1945) on topographic maps
• all coordinates are in metres: note that decimal places are a result of GIS precision, but are meaningless

The most common system in Canada is the Universal Transverse Mercator (UTM): the basic details are:

• The World is divided into 60 N-S zones, each 6 degrees longitude wide (Prince George is in zone 10)
• Canada covers 16 zones, each with their own Central Meridian (CM)
• Each zone varies in width: 668 km at equator, 115 km at 80N
• Coordinates are given by 'eastings' (first) and then 'northings'
• Easting (6 digits): relative to CM = 500,000 (metres)
• Northing (7 digits): relative to Equator = 0 (metres)
• Equator to Pole = 10,000,000 metres

UTM works well for local areas and within a zone City of Prince George less handy for the whole of BC

UTM data for multiple zones

For analysis of areas that cross 2 or more zones, data must be re-projected into just one zone (as if they were in that zone) ..topographic map example ... although eastings coordinates will go further away from the central meridian (500,000) and potentially below 0  .. and area distortion will increase.

View the coordinates on mapplace website to see UTM coordinates over multiple zones

Provinces slightly more than one UTM zone may use a modified system:

Alberta crosses 2 zones 11-12 (110-120W) - and uses '10TM' (one 10 degree zone centred on longitude 115 W)

Saskatchewan crosses 2 zones 12-13 (102-110W)

Manitoba crosses 2+ zones 13-15 (89-102W)

BC crosses 5 zones 7-11 (114 - 139W)     UTM zones in BC   UTM zones / map sheets

For this reason, BC chose the Albers equal area projection for storing and displaying provincial data.

5. (BC) Albers Equal Area projection

Albers is a conic equal-area projection usually with 2 standard parallels:

BC Albers projection   BC GIS website

• Central meridian: -126.0 (126:00:00 West longitude)
• Latitude of projection origin: 45.0 (45:00:00 North latitude)
• First standard parallel: 50.0 (50:00:00 North latitude)
• Second standard parallel: 58.5 (58:30:00 North latitude)
• False easting: 1000000.0 (one million metres)
• False northing: 0.0 (northing coordinate at 45 degrees north)
• coordinates are in metres (decimal places are from GIS, not meaningful in most cases)

This gives one set of coordinates with zero area distortion for any part of BC Albers map and grid

Other locations might use a similar projection (North America) but with a different centre and parallels

The Canada Albers Equal Area projection:  central meridian at 96W, origin at 40N and standard parallels at 50, 70N

Yukon : Yukon Albers Projection - note the different coordinates and standard parallels

Canada: Lambert conformal conic - note different parallels again and it is conformal (shape), not equal area

6. GIS georeferenced coordinates (summary)

So you are likely to see coordinates in any of these three systems for BC GIS data:

• Geographic
• UTM (zones 7-11)
• Albers

What approximate values would Prince George be in each of these three systems above? see lrdw - imap website

Identifying coordinate systems

Note that you can receive data without proper documentation, and might need to guess (and which is x and y)

In software:

Define projection:  relabels the data (adds 'metadata') but changes ONLY the description (not the data)

Reproject:              changes the data projection coordinates (e.g. from UTM to Albers, or NAD27 to NAD83)

Ideally all layers should be maintained using the same coordinate system .. although if properly defined (labelled), software can reproject layers 'on the fly'.. otherwise layers will not overlay and in any case cannot be combined for analysis.

Q: If layers are from the 3 different systems above, and not properly defined, how would they locate relative to each other?

Source: http://www.gis.unbc.ca/courses/geog300/lectures/lect3/index.php