GIS Data: Vector, Projection, and ArcGIS Units

• Discrete data represents distinct features that have clear boundaries; examples include cities, rivers, and roads.
• Continuous data represents phenomena with gradual change over space, like elevation, temperature, and rainfall.

Vector Attribute Fields and Link to Geometry

• Vector data represents features as points, lines, or polygons.
• Attribute fields are tables that store data about features.
• Each row in the attribute table corresponds to a feature on the map.

Projection Surfaces and Shapes

• Projection surfaces are methods to flatten the Earth’s 3D surface onto a 2D map.
• Plane (Azimuthal) projections are useful for polar regions.
• Cone (Conic) projections are best suited for mid-latitudes, such as the U.S.
• Cylinder (Cylindrical) projections are best for equatorial areas.
• Projection shapes preserve area (equal area), shape (conformal), distance (equidistant), or direction (azimuthal).

ArcGIS Pro Map Units and Display Units

• Map units are units of the data’s coordinate system (e.g., meters, feet, degrees).
• Display units are how units are shown in ArcGIS, like degrees even if the data is in meters and are set in the Map Properties window.

Vector and Raster Data Model and Data Types

• Vector models use points, lines, and polygons and are used for cities (point), roads (line), land parcels (polygon).
• Raster models use a grid of cells with values.
• Raster data types include elevation (DEM), satellite imagery, and temperature.

Understanding the ArcGIS Pro Window

• The Table of Contents (TOC) lists all layers in the map.
• Layers are individual datasets shown on the map.
• Other elements include the Catalog Pane (manage data), Attribute Table (view data fields), and Map View (display).

Shapefile Components

• A shapefile is composed of several files.
• .shp files store geometry (actual shapes).
• .shx files are index files.
• .dbf files store attribute tables.
• Additional files include .prj (projection) and .sbn/.sbx (spatial index).

Topological and Non-Topological Data

• Topological data maintains spatial relationships like connectivity, adjacency, and containment; for example, ensuring polygons share a common boundary without gaps.
• Non-topological data does not enforce relationships and features are independent, like a shapefile where two lines cross without awareness of each other.

Earth Surfaces

• The geoid is the shape of the Earth defined by gravity, and is irregular like mean sea level.
• The topographic surface is the actual Earth's surface, including mountains and valleys.
• The ellipsoid is a smooth, mathematical surface used for calculations and is a simplified Earth model.

Distortion Created During Map Projection

• Since the Earth is 3D and maps are 2D, projections distort shape, area, distance, and/or direction.
• No projection preserves all four; the choice depends on what is most important (e.g., navigation vs. land area comparison).

False Origins in Coordinate Systems

• A false origin is an artificial origin set to avoid negative coordinate values and is common in projected coordinate systems like UTM.
• UTM sets a false easting of 500,000 meters to ensure all x-values are positive.

Raster Cell Resolution

• Raster cell resolution refers to cell size in raster data (e.g., 10x10m).
• Smaller cells mean higher resolution and more detail.
• Bigger cells mean lower resolution and faster processing, but less accuracy.

Units in Projected vs. Geographic Coordinate Systems

• Projected Coordinate Systems (PCS) use linear units like meters and feet; an example is UTM or State Plane.
• Geographic Coordinate Systems (GCS) use angular units like degrees, minutes, and seconds; an example is WGS 1984.

Mapping Symbology

• Categories use different symbols/colors for distinct classes; an example is land use types like residential, commercial, or agricultural.
• Quantities show numeric differences using graduated colors/sizes; an example is population density.
• Normalization is dividing one value by another for fair comparison; for example, population per sq. km instead of total population.

Overlay (Relationship) Tools in ArcGIS

• Intersect keeps features common to both inputs.
• Union merges features from both inputs and keeps all.
• Identity combines input with overlay, keeping all input features.
• Clip cuts input data to match the shape of an overlay layer.
• Erase removes features within the overlay area from the input.

Georeferencing, Data Projections, and Digitising

• Georeferencing assigns real-world coordinates to an image or dataset; for example, aligning a scanned map to latitude/longitude values in ArcGIS.
• Data projections transform geographic coordinates (lat/long) to projected coordinates (like meters).
• Data projections make distance and area calculations more accurate for specific regions.
• Digitizing converts features on a map/image into digital vector data by tracing them.

Distortion in Aerial Photographs

• Distortion is caused by relief displacement (higher objects appear tilted outward from the center), camera angle (tilt) which skews features, and lens distortion which causes curved image edges.
• Orthorectification is used to correct distortion in aerial photographs.

Ellipsoid Measurements

• The Earth is not a perfect sphere, it's flattened at the poles and an ellipsoid approximates this shape.
• WGS 1984 is a common ellipsoid used in GPS.
• GRS 1980 is used in NAD 83.
• Semi-major axis (equatorial radius) and flattening define ellipsoids.

Distance Options in the Measure Tool in ArcGIS Pro

• Planar measurement is straight-line and 2D, best for projected data.
• Geodesic measurement follows the Earth’s curve and is best for geographic data.
• Loxodromic measurement follows a constant compass direction (rhumb line).
• The great Elliptic measurement is more accurate over long distances.

Lines of Latitude and Longitude

• Latitude (parallels) are horizontal lines measuring N/S of the Equator (0° to 90°).
• Longitude (meridians) are vertical lines measuring E/W of the Prime Meridian (0° to 180°).
• Latitude and longitude together form the grid used in geographic coordinate systems.

Polygon Feature Relationships

• Adjacency occurs when polygons share a boundary (e.g., neighboring countries).
• Containment occurs when one polygon completely contains another (e.g., a lake inside a park).
• Overlap occurs when two polygons partially cover the same area (e.g., overlapping zoning areas).

Meaning of Quantity and Category Data

• Category data (qualitative) describes what or type; examples include land use type, soil class, and tree species.
• Quantity data (quantitative) describes how much or how many; examples include population, elevation, rainfall, and income.