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Projections and geoprocessing

key terms​

  • prime maridian: 0° longitude
  • equator: 0° latitude
  • datum: reference point for measuring locations on Earth
  • projection: algorithm to convert 3D Earth to 2D map

All projections have a coordinate system

!tl;dr GCS is sphrere, PCS is flat map.

Geographic Coordinate SystemProjected Coordinate System
Earth assphere 🌍flat map 🗺️
Unitsdatum, angular unit of measure, & central meridiancartisian coordinates, latitude of origin & central meridian
Measured inangles (44deg )meters (22m, 33m)

coordinate system

GCS vs PCS

!important take earth and squish it inot a flat map -> distortion

Properties that get distorted from 3D to 2D:

  • shape
  • area
  • distance
  • direction

Projection types:

  • equirectangular (default in ArcGIS): equal, constant distances between parallels and meridians.
  • mercator: angles preserved, good navigation, optimized for calculation speed, standard for web mapping
  • mollweide: relative areas preserved, shape distorted
  • universal transverse mercator (UTM): 60 zones, each 6° wide, good for local mapping

How does Greenland appear in each of these projections?

  • equirectangular: meridians converge at poles -> more stretched out at poles -> Greenland looks bigger than it is
  • mercator: Greenland looks huge, as big as Africa.
  • mollweide: Greenland looks normal, but Africa looks smaller than it is.

Goal​

Find all countries that overlap with a bounding box on a map of North and South America.

Steps​

  1. Create a bounding box over target region
  2. Use Analysis Tools to find overlapping countries
  3. Run Union tool to combine features
  4. Join attributes to get country data

note: need to set environment settings to control tool behavior: mainly the processing extend needs to be set to the extent of layer > the bounding box.

Key points:

  • Analysis tab contains geoprocessing tools
  • Union tool combines features (like set theory)
  • Can join attributes of features
  • Environment settings control tool behavior
  • Results show all countries intersecting the box

toolbox

Goal​

We've various waterbodies and we want to find out which counties they are in.

Tool: spatial join.

  • connects geographic features based on location

Steps​

  1. Select watershed features as target
  2. Perform spatial join with country boundaries
  3. Define one-to-many relationship (watershed to countries)
  4. Configure field mappings and rename attributes
  5. Review results in attribute table

Key points:

  • tool: Spatial Join tool connects geographic features based on location.
  • relationship: 1 to many One waterbody can overlap multiple counties.
  • Field mapping allows customizing output attributes.
  • Attribute table shows final joined data.
  • Can rename fields for clarity (e.g., "Name" to "County").

spatial join

✝️symbology

Concerned with how we choose to display GIS data.

  • Water with blue line 🔵
  • Counties with red outline 🔴
  • Watershed with green fill 🟩
➕ Create a new feature class from selection
  1. create new map document
  2. add data - NDHFlowline
  3. change symbology to style of water 🤽‍♂️🤽‍♂️.
  4. Add navarro_boundary
  5. Select flowline that intersects location of boundary
  6. Remove selection of flowline that intersects the ocean.
  7. Make the only selectable layer.
  8. Save the new layer (export features).
  9. Store layer in separate geodatabase to compartmentalize source data and analysis results.
  10. Clear selected features.