spatial reference systems

georeferencing

= the process of describing location, using place names, addresses, directions, or some sort of coordinate system

• When combining data sets together, your data must be georeferenced in the same way

two types of positioning

1. relative positioning = giving location in reference to one thing or another → what we do when we give people directions → basically how we talk about locations and how to get there

2. absolute position = the use of coordinate systems that superimposes a grid over an area

• local grid = grid format can be used universally and what is on the grid depends on where you are (ex: a city map or a world atlas)

• global (geographic) grid = tied to the earth, every single point of the earth can be described uniquely and that description world never change → used universally

  • latitude and longitude = system of uniquely positioning points

  • UTM gird

longitude and latitude

 = a system for uniquely locating points

graticule

= the imaginary network of lines of longitude and latitude that are measured using a sexagesimal system of degrees, minutes, and seconds

sexagesimal system (base 60)

= measures in degrees, minutes, and seconds -> base 60 because there are 60 minutes in a degree and 60 seconds in a minute

latitude

= the parallels: the horizontal lines that go from east to west

• the origin is the equator

• parallels are labelled north or south of the equator from 0 degrees to 90 degrees north/south

• parallels get shorter and shorter toward the poles, until they are at the poles

• the north pole is 90 degrees, the equator is 0 degrees, Vancouver is 49 degrees and 13 minutes N latitude

longitude

= the meridians: vertical lines that go north to south

• the origin is the prime meridian, which goes through Genrich, England → the origin can be any line of longitude, but the agreed point is the prime meridian

• meridians are labelled west or east of the prime meridian from 0 degrees to 180 degrees W/E of the prime meridian

• meridians are always the same length and converge at the poles

• Vancouver is west of the prime meridian and 123 degrees and 6 minutes W of the prime meridian

decimal degrees (base 10 system)

• we convert the base 60 system into base 10 when we use computers and GIS programs

• north and east are represented as (+); south and west are represented as (-)

• the conversion is basically the degree + the minutes/60 and the (+)/(-) depends on direction

convert 49 degrees 13 minutes north and 123 degrees six minutes west to decimal degrees

how much space is between each degree of latitude and longitude

latitude = 111km from each degree of latitude

longitude = 111km between any 1 degree of longitude along the equator (as you move away from the equator the longitude lines get small and so does the distance as you move closer to the north/south pole)

ways to measure direction of longitude and latitude lines

Great circle = results from a plain slicing through the center of the earth (ex: the equator is a great circle) -> to be a great circle, it needs to cut through the center of the earth

• All meridians and the equator are great circles and there are many more

• The arc of the great circle represents the shortest path between two points

  • Very useful for airline navigation

• The shortest distance then is not necessarily a line of constant direction

Rhumb lines = great circle route approximations -> using a series of short lines of constant direction to approximate the great circle route - it is a series of lines of constant direction

• What airline navigation does when creating it's routes

Small circles = results from a plain slicing through the earth (doesn't cut through the center of the earth)

• The arcs of these circles do not measure the shortest path between two points

UTM (Universal Transverse Mercator) grid

= a geographical global grid commonly used instead of latitude and longitude → used for large scale mapping (ex: reference mapping), topographical maps, and survey maps

• divides the earth into 60 6 degree wide vertical strips between 80 N and 8 S latitude

  • each vertical strip has its own central meridian and you measure out from there → it is measured in meters → vertical strips are called zones numbered 1-60

• the zones are divided into horizontal bands → each band is 8 degrees tall from north to south

• this creates a grid made of grid zones → each grid zone is subdivided into 100km/100,000m squares called quadrilaterals

• a location on earth is referred to by its zone number, band number, and coordinate (made up from an easting and a northering measured in meters)

ex: British Columbia is part of zones 7-11

• mapping BC with a UTM uses zone 10 because it covers most of BC

scale and the three ways to express scale

= expresses distance on the map to distance on the earth

1. Representative fraction = the relationship between map distance and earth distance, represented by a ratio

• It is unitless

• It is hard to measure distances with representative fractions

  • You can estimate distances but it is not easy to use

• Invalid if a map is reduced or enlarged in size

  • Good for maps where you can't change it's size - i.e physical maps

• Ex: 1:100,000

2. Verbal scale = added units to represent the distance - tells you what to do when you are using a representative fraction

• Ex: 1 cm = 1km

• Easier to estimate

• Invalid if the map is reduced or enlarged in size

• Hard to measure still

3. Bar scale = adding a bar to represent the distance - its like putting a ruler on the map

• Will still be valid if the map changes in size

• Easy to estimate

• Most preferred use of scale

3 types of north

1. True north = defined by the axis the earth rotates

• Ex: the north pole

2. Grid north = based on the coordinate system you are using

• Will be similar to true north

3. Magnetic north = the direction a compass takes based on the magnetic pole - the magnetic pole is not at true north and it also changes from north to west at 40km a year

• If a map is designed for compass users then you will need to include the multiple expressions of north

• Magnetic declination = the difference between true north and magnetic north

do you have to include north every time?

• no → depends on context

  • reference maps need inclusion of north; thematic maps of familiar areas might not need it

  • don’t need north if using a graticule, but then you need to label the graticule where longitude and latitude is

do you need to include a scale every time?

depends on the map

• reference maps need all three scales because they are used for navigation

• thematic maps don’t need to have a scale → if the area mapped is recognizable to all map readers and distance is not relevant to the theme you prob don’t need it

• when in doubt add a scale