Spatial data, map formatting and projections Flashcards
Definition of a map
A map is a flattened, filtered and scale model balancing reality with generality
Scaled: everything is smaller in terms of size
Filtered: not everything in the real world is represented
Flattened: a map is a 2D representation of a 3D world
Why do we make maps?
To organise and summarise information
To communicate information
To investigate patterns
Definition of GIS
*GIS: a geographic information system is a computer software program that allows creating, modifying, analysing and visualising spatially explicit data Location: what is at...>? Trends: what has changed since...>? Patterns: what spatial patterns exist? Modeling: what if…?
Spatial data: definition and types
Information about LOCATION and SHAPES of geographical features
Geographically explicit data
Location: where you measure something
Attributes: what measured and its value
Geo-referenced data: location and attributes
Spatial data: Raster data definition
Grid cells in an array. Each cell holds the value of the attribute that occurs in that location (usually just 1 attribute)
Pixels and photos
Formats: GRID or ERDAS image file
GIS operates on pixel locations - e.g. satellite images, medical images
Attributes: amount and type of reflected light, amount of vegetation green-ness
Spatial data: Vector data definition
: lines and nodes can be represented by point, lines or polygons. This is the form that holds the data in the manner that is most like traditional paper map
A created image
Vector formats
Shapefiles: simplest and most transportable
- always need .shp (geographic data), .dbf (the attribute data), .shx (stores what is needed to access the .shp data and links it to the attribute table)
Feature classes, SDC, Coverage
How to save maps
ALWAYS check “pathnames: store relative pathnames to data solves” so that data moves with file
mxd file just saves the symbology not data
No brackets in the name, just underscores
Raster v vector
Different types of data storage
Raster includes locations are linked with limited associated attributes
Raster allows some compression (e.g. JPEG)
Vector typically more efficient to store
Line between vector and raster is blurring
ArcGIS can handle both types of data, but is primarily a vector system
3 basic attributes of maps
Scale
Projection
Symbolisation
THEY DESCRIBE THE MAP’S POSSIBILITIES AND LIMITATIONS - do not confuse with features of a map layout
Geodatabase
A file you can make to store other files
A container used to hold a collection of datasets (GIS features, tables, raster images etc)
Not good for use if overseas
Scale
Formally: how does distance on map correspond to distance on the ground
Impact: controls level of detail you can show
Ratio: measure on the ground : measure on the map
Large v small scale map
Large scale map: 1:5,000
Shows one small area with lots of detail e.g. a village
Small scale map: 1:50,000,000
Showing a large area with few spatial entities being displayed
World or country map
Fine scale: small area where you can see fine/small details
Coarse scale: large area where you can only see coarse/large important details
Types of scales
Ratio scale: 1cm = 4km Verbal scale: one inch equals four km Graphic scale: scale bar Generally safer to use Always remains relative to the map - don’t need to update or change
Scale Bar
Show relative proportion of the map to the real world size
Like the north arrow it is generally okay to leave this element out if showing a country map like the US but it is important to use when showing fine scale maps of study areas
International maps
*Maps made outside the US should have scale bar in metres not miles
For maps in US depends on audience
Scientific audiences: metres
General public: miles
Longitude and Latitude
a geographic coordinate system - UNPROJECTED
Latitude
Latitude: Y The distance from the equator along the Y axis Expressed in degrees South pole: 90S North Pole: 90N Equator: O 90 is the maximum value
Longitude
The distance from the prime meridian along the X axis
In degrees
Prime meridian: 0
Halfway around the world = 180 degrees
Only 1 180 degree line but looks like 2 lines on a map, but it is the same line
180 is the max value
Distance errors
Cylindrical projections are exact in the equator but increase the measurement error with increasing latitude
*all projections introduce some error - distance, area, direction, shape
A mile or metre stay the same wherever you are in the world but degrees change
Projection: definition
**Projection: to transform the curved, 3D surface of the planet into a flat 2D plane
Earth measurements and mathematical formulas determine geodetic ellipsoids and datums
2 steps
Shrink the earth into a sphere/globe
Try to minimise the distortions with projection
IT IS WISE TO STICK WITH A PROJECTION AND STICK WITH IT THROUGHOUT ANALYSES
Types of projections
Conformal projections: preserve local shape (good for measuring angles, navigation)
Equal area projections: preserve areas
Equidistant: preserve distance
BUT NO PROJECTION PRESERVES ALL FEATURES
Projections: developable surfaces
Developable surfaces The surface that opens up to give the map 3 options: plane, cylinder, cone Conic: good for mid-latitudes Cylindrical: good for equatorial regions Azimuthal/plane: good for polar regions
Metadata
The data on the data
Often in layer property section
WGS_1984: means it is NOT a projected file - just has a coordiante system applied to a data
UTM475_WGS_1984 means it is projected and has a coordinate system
Common projections
*Universal Transverse Mercator (UTM)
Coordinates in metres (northing, easting)
Trans mercator means mercator projection in a narrow strip around particular lines of longitude
60 zones starting at international date line
All coordinates are positive and in the hundred thousands or millions of metres for most of north america
Important to give northing, easting and zone
*Lambert Conformal Conic projection
Good for east-west states (Tennessee) but not for north-south long states
*State planes coordinate system projections
Can be measured in metres or in feet
Divides US into 120 zones with 3 conformal projections
Lambert conformal conic: for long east-west (TN or Kentucky)
Transverse mercator: long N-S - Illinois, Vermont
Hotine Oblique Mercator projects for Alaska
Equirectangular projection
Geographic system (lat-long) which is based on angles measured on a sphere is not a valid measurement on a plane Lat-long are called unprojected because they define positions on a curved sphere/ellipsoid and have not yet been thrown on projected onto a plane
