+ - 0:00:00
Notes for current slide
Notes for next slide

Working with Geospatial Data

Projections

Ben Baumer

SDS 192
April 10th, 2020
(http://beanumber.github.io/sds192/lectures/mdsr_geo_06-projections.html)

1 / 16

Projections

2 / 16
3 / 16

Principles

https://en.wikipedia.org/wiki/Map_projection

"All projections of a sphere on a plane necessarily distort the surface in some way and to some extent."

4 / 16

Principles

https://en.wikipedia.org/wiki/Map_projection

"All projections of a sphere on a plane necessarily distort the surface in some way and to some extent."

  • Properties:
    • Area
    • Shape
    • Direction
    • Bearing
    • Distance
4 / 16

Principles

https://en.wikipedia.org/wiki/Map_projection

"All projections of a sphere on a plane necessarily distort the surface in some way and to some extent."

  • Properties:
    • Area
    • Shape
    • Direction
    • Bearing
    • Distance
  • Every projection represents a trade-off in accuracy among these proporties
4 / 16

Distortion

5 / 16

Projections

6 / 16

Projections

6 / 16

Projections

Important!!: Not OK to pretend like projections don't matter

6 / 16

Mercator (preserves angles)

7 / 16

Gall-Peters (preserves area)

https://en.wikipedia.org/wiki/Gall%E2%80%93Peters_projection#Controversy

8 / 16

Albers Equal Area

9 / 16

Lambert Conformal Conic

10 / 16

PROJ.4 strings and EPSG

library(sf)
library(macleish)
# GPS, Google Earth, Leaflet
st_crs(4326)
## Coordinate Reference System:
##   User input: EPSG:4326 
##   wkt:
## GEOGCS["WGS 84",
##     DATUM["WGS_1984",
##         SPHEROID["WGS 84",6378137,298.257223563,
##             AUTHORITY["EPSG","7030"]],
##         AUTHORITY["EPSG","6326"]],
##     PRIMEM["Greenwich",0,
##         AUTHORITY["EPSG","8901"]],
##     UNIT["degree",0.0174532925199433,
##         AUTHORITY["EPSG","9122"]],
##     AUTHORITY["EPSG","4326"]]

https://epsg.io/4326

11 / 16

Checking projections

macleish_layers %>%
  pluck("buildings") %>%
  st_crs()
## Coordinate Reference System:
##   User input: EPSG:4326 
##   wkt:
## GEOGCS["WGS 84",
##     DATUM["WGS_1984",
##         SPHEROID["WGS 84",6378137,298.257223563,
##             AUTHORITY["EPSG","7030"]],
##         AUTHORITY["EPSG","6326"]],
##     PRIMEM["Greenwich",0,
##         AUTHORITY["EPSG","8901"]],
##     UNIT["degree",0.0174532925199433,
##         AUTHORITY["EPSG","9122"]],
##     AUTHORITY["EPSG","4326"]]
12 / 16

Units really matter!!

macleish_layers %>%
  pluck("buildings") %>%
  st_bbox()
##      xmin      ymin      xmax      ymax 
## -72.68251  42.44104 -72.67923  42.44919
13 / 16

Projecting geospatial data

buildings_3857 <- macleish_layers %>%
  pluck("buildings") %>%
  st_transform(3857)
buildings_3857 %>%
  st_bbox()
##     xmin     ymin     xmax     ymax 
## -8090979  5227276 -8090614  5228505
14 / 16

Oops!

library(leaflet)
leaflet() %>%
  addTiles() %>%
  # No error, but puts buildings at North Pole!
  addPolygons(data = buildings_3857, popup = ~name)
15 / 16

Projections checklist

  • : Are you using leaflet?
    • Project everything in EPSG 4326
16 / 16

Projections checklist

  • : Are you using leaflet?
    • Project everything in EPSG 4326
  • : Are you using ggplot2?
    • Choose a projection scheme that is appropriate for your data
    • Project everything in that scheme
16 / 16

Projections

2 / 16
Paused

Help

Keyboard shortcuts

↑, ←, Pg Up, k Go to previous slide
↓, →, Pg Dn, Space, j Go to next slide
Home Go to first slide
End Go to last slide
Number + Return Go to specific slide
b / m / f Toggle blackout / mirrored / fullscreen mode
c Clone slideshow
p Toggle presenter mode
t Restart the presentation timer
?, h Toggle this help
Esc Back to slideshow