Lecture 2: GIS Fundamentals  Data layers  Feature classes  GIS data properties  Data types –vector and raster  Scale  Accuracy and precision

Q&A May I record the lectures for my own review? –Yes, but I don’t want to find myself on Youtube Can I download the slides ahead of time? –Usually: May I use my computer to take notes? –I don’t mind; class opinion? Any others?

Spatial data are divided into layers (themes).. as also in 20 th century mapping (transparencies) overlaying map layers is what drove GIS development 1. GIS layers

Pre-GIS Layers

Layers: 1

Layers: 2 PG: 12

Layers: 3

Layers: 4 (6)

Layers out of order

Layout View

Topographic map example of print layers Forests, Contours, Urban, Buildings, Rivers, Streams, Lakes, Roads (major/minor), Trails, Labels Total = ~16 … separated for color printing and clarity – it’s the same data layers for GIS

2. Feature classes (geometry) Points: have ‘no dimensions’ (no width, length, or height) Lines: have length (recorded as connected points - vertices) Polygons: have area (recorded as connected, closed vertices) Layers do not mix feature types (this figure shows 3 layers)

Points: No Dimensions? Dimensions may be stored as attributes In this case, spot area (hectares)

Points: No Area? “Point” = center point Display = actual area Why?

Lines: No Width? Road width (Victoria, Australia) Line = centerline = dimensionless Width = attribute = user defined/calculated

Polygons: Finally, dimensions? Polygons have area by definition Dimensions depends on location, projection Area = attribute stored with file

3. Properties of GIS data  Spatial (x,y location) = ‘where is it ?’  Attributes (multiple) = ‘what is it’ ?  [Pattern] = ‘how are they related’ ?

Location and Attributes

Spatial location – ‘where is it’ ? x (easting), y (northing) [z-elevation] coordinates e.g. latitude / longitude (degrees, minutes, seconds) or UTM coordinates (metres)

Attributes Attribute data: allows us to ask the question … "what is it ?" > Every layer has an associated table > These are linked to spatial location by a code number  Attributes are stored in columns as items  Rows display the attributes for each feature = records  Entries may be text strings, integers, float (decimal) or dates Item Record ->

Attributes (Items) One Record Seven Items

GIS file formats – Esri shapefiles One “shapefile” (GIS layer) contains multiple files:.shp Spatial data e.g. rivers.shp.shx Shape index link file e.g. rivers.shx.dbf Attribute data e.g. rivers.dbf Also may include:.prj Projection file.sbn and.sbx.. these help optimize spatial queries

Types of questions a GIS can answer – resulting from spatial location, attributes and patterns a.Location: WHAT exists here "What is at this location ?" e.g. Digsafe, forest attributes. b. Condition: WHERE are specific conditions Where are all the pine dominated stands ? c. Trends: WHAT HAS CHANGED (over time) How far has the river bank receded in the past 10 years ? d. Patterns: HOW are features related "How does proximity to salmon streams affect the number of bear attacks"; e. Modelling: WHAT IF..? What if the climate warmed by 2 degrees? (e.g. effect on habitats)

4. GIS data types: raster and vector A. Pixel based (grid) - faster analysis / overlay Why? B. Feature based - attribute management and map output ’Reality’ …….

Raster and Vector Data 48 Records 48 Locations 1 Record per feature 1-∞Locations

5. Scale Scale: the amount of reduction from reality This is written as a ratio e.g. 1:1,000 or 1:1,000,000 Reality = 1:1 The larger the number on the right, the smaller the scale i.e. an area is shown in less detail and a larger area is shown (1:1,000,000 is a smaller scale than 1:1,000) More intuitive to say coarse scale and fine scale As the scale increases, the GIS data can contain more details Spatial data are intended for a specific scale / range of scales

Scale bars: map output For map output, scale bars are more common, e.g. GIS software, like ArcGIS or Google Maps can generate these automatically.. but not always well: … pick logical suitable units

6. Precision and Accuracy Accuracy : how close is the value to the actual value (data quality) Precision : how exactly can a value be specified (based on scale) On printed maps, precision was equivalent to ~ 0.5mm (= 25 metres at 1:50,000 … or 125 metres at 1:250,000) This is more complex with zoomable GIS / online mapping

Precision and Accuracy GIS software uses ‘double-precision’ – capable of storing 15 digits … 6 decimal places (of metres) or 10 (latitude/longitude) e.g or In most cases, this level of precision is not warranted by the data e.g. trail lengths, locations, cut block area … to the nearest mm ? Beetle survey to 1/10 th millimeter:

Precision and Accuracy: Google According to Google: Boundary line: 119°59'59.76"W Boundary cut: 120° 0'4.25"W Nominal boundary: 120°W? Legally: “Line between survey posts”

Summary and Questions Layers Points, Lines and Polygons –Display vs. Analysis Location and Attributes –Locations:.shp Attributes:.dbf Vector and Raster Data Scale Precision and Accuracy

Napoleon’s March to Moscow, Charles Joseph Menard, 1869