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Week2 - ArcGIS Basics

Using ArcMap for data exploration

  • right click 'Identify' to look at attributes for one item

  • or multi select -> Right click on layer -> open attribute table, to see attributes of the multiple selection

  • Add a basemap for reference and change transparency (Layer Property -> Display) of the new layer to explore to overlay on top of basemap

  • Right click on layer -> Data -> item description to read metadata about the file

  • Layer property

    • scale dependent rendering: don't show when scale > x
    • Choose grouping and color ramp;
    • Definition query: e.g. "CumulativeArea.TotDASqKM > 5"
    • Labels - choose label field

  • Select by attribute:

    • query:
      • use "Get Unique Values" to help find value options for query
      • string matching GNIS_NAME LIKE '%Sacramento River'
    • layer -> zoom to selection

  • Calculate geometry

    • to find sum of length of rivers named X: select by atttribute -> create a new column -> calculate geometry -> statistics on the column

  • Select by location

    • after selecting some regions -> Select by location -> can find things in another layer that intersects/within/centroid lands in/ + add a radius... with the current selection

  • Export the selected data for future

Projections

  • Datum - fixed reference point for coordinates
  • Geographic coordinate systems
  • Projected coordinate system
    • conformal - preserves local shape
    • equal area - preserves area
    • equidistant - preserve distances between points
  • common projections
    • equirectagular: equal constant distance between parallels and meridians
    • Mercator (conformal): angles preservation, good for navigation, standard for web, optimized for calculation speed
    • Mollweide (equal area): relative area preserved but shape distorted
    • Universal Transverse Mercator

Geoprocessing

  • Geoprocessing tool box:

    • Union: join two features (e.g. take NA map and SA map into a hemisphere map)

  • Environment variables

    • change projections
    • Processing extent: limit the area to do analysis

  • Spatial Join Tool

    • ArcToolbox -> Analysis Tools -> Overlay -> Spatial Join

    • JOIN_ONE_TO_ONE: use merge rule (e.g. join counties to watershed, but each watershed may go across multiple counties)

    • JOIN_ONE_TO_MANY: each watershet + county is a new row

    • example uses:

      • use it for summarizaiton
      • which road cross a river
      • join multiple layers (e.g. electoral district + water + utility )