Working with Geospatial Data

class: center, middle, inverse, title-slide

# Working with Geospatial Data
## Projections
### Ben Baumer
### SDS 192</br>April 10th, 2020</br>(<a href="http://beanumber.github.io/sds192/lectures/mdsr_geo_06-projections.html" class="uri">http://beanumber.github.io/sds192/lectures/mdsr_geo_06-projections.html</a>)

---

class: center, middle, inverse
# Projections

---
background-image: url(http://gisedu.colostate.edu/webcontent/nr505/2012_Projects/Team6/images/GIS_concepts/Figure2_proj.PNG)
background-size: contain
---
## Principles

.footnote[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

---
## Distortion

.center[![](https://s-media-cache-ak0.pinimg.com/564x/ee/b8/ba/eeb8bafd454afce39587fdb960a55e85.jpg)]

---
## Projections

.pull-left[

![](https://media.giphy.com/media/wHASLyWK4zj5OhRCnv/giphy.gif)

]

.pull-right[

- It's complicated (non-trivial math)

- There is no one "best" projection

- Explore: (https://bl.ocks.org/mbostock/raw/3711652/)

]

.center[**Important!!: Not OK to pretend like projections don't matter**]

---
## [Mercator](https://en.wikipedia.org/wiki/Mercator_projection) (preserves angles)

![](https://upload.wikimedia.org/wikipedia/commons/f/f4/Mercator_projection_SW.jpg)

---
## [Gall-Peters](https://en.wikipedia.org/wiki/Gall–Peters_projection) (preserves area)

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

![](https://upload.wikimedia.org/wikipedia/commons/3/34/Gall%E2%80%93Peters_projection_SW.jpg)

---
## [Albers Equal Area](https://en.wikipedia.org/wiki/Albers_projection)

![](https://upload.wikimedia.org/wikipedia/commons/4/4f/Kavraiskiy_VII_projection_SW.jpg)

---
## [Lambert Conformal Conic](https://en.wikipedia.org/wiki/Lambert_conformal_conic_projection)

![](https://upload.wikimedia.org/wikipedia/commons/6/67/Lambert_cylindrical_equal-area_projection_SW.jpg)

---
## PROJ.4 strings and EPSG

```r
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"]]
```

.footnote[https://epsg.io/4326]

---
## Checking projections

```r
macleish_layers %>%
  pluck("buildings") %>%
  st_crs()
```

---
## Units really matter!!

```r
macleish_layers %>%
  pluck("buildings") %>%
  st_bbox()
```

```
##      xmin      ymin      xmax      ymax 
## -72.68251  42.44104 -72.67923  42.44919
```

---
## Projecting geospatial data

```r
buildings_3857 <- macleish_layers %>%
  pluck("buildings") %>%
* st_transform(3857)

buildings_3857 %>%
  st_bbox()
```

```
##     xmin     ymin     xmax     ymax 
## -8090979  5227276 -8090614  5228505
```

---
## Oops!

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

<div id="htmlwidget-4454f765728c3308880a" style="width:504px;height:360px;" class="leaflet html-widget"></div>
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href=\"http://creativecommons.org/licenses/by-sa/2.0/\">CC-BY-SA<\/a>"}]},{"method":"addPolygons","args":[[[[{"lng":[-8090763.01964324,-8090753.2624485,-8090753.16285252,-8090762.92003987,-8090763.01964324],"lat":[5228385.75520565,5228385.89075171,5228378.66841783,5228378.53287197,5228385.75520565]}]],[[{"lng":[-8090757.68467349,-8090757.06645846,-8090756.46553217,-8090755.90015348,-8090755.38750116,-8090754.94315188,-8090754.58060699,-8090754.31088221,-8090754.14217298,-8090754.07960543,-8090754.12508063,-8090754.27721681,-8090754.53139137,-8090754.87988133,-8090755.312098,-8090755.81490868,-8090756.37303575,-8090756.96952079,-8090757.58623994,-8090758.20445451,-8090758.80538039,-8090759.37075877,-8090759.88341093,-8090760.32776019,-8090760.69030523,-8090760.9600303,-8090761.12873992,-8090761.19130792,-8090761.14583318,-8090760.99369741,-8090760.73952316,-8090760.39103337,-8090759.95881671,-8090759.45600588,-8090758.89787853,-8090758.30139309,-8090757.68467349],"lat":[5228370.42135256,5228370.37571069,5228370.22301634,5228369.96790906,5228369.61814019,5228369.18433728,5228368.67968123,5228368.11950579,5228367.52083163,5228366.90184915,5228366.28136584,5228365.67823477,5228365.11078175,5228364.59624855,5228364.15026897,5228363.78639386,5228363.51567936,5228363.34635098,5228363.28355366,5228363.32919546,5228363.48188959,5228363.73699652,5228364.08676497,5228364.52056741,5228365.02522302,5228365.5853981,5228366.18407203,5228366.8030544,5228367.42353778,5228368.02666908,5228368.59412244,5228369.10865607,5228369.55463611,5228369.91851166,5228370.18922652,5228370.35855515,5228370.42135256]}]],[[{"lng":[-8090837.62291594,-8090825.66734653,-8090825.76979257,-8090837.72534232,-8090837.62291594],"lat":[5228090.14215192,5228090.06346621,5228074.38477314,5228074.46345859,5228090.14215192]}]],[[{"lng":[-8090829.35098241,-8090820.32542465,-8090820.82798886,-8090829.85352349,-8090829.35098241],"lat":[5228074.40834434,5228074.22154282,5228049.76148641,5228049.94828697,5228074.40834434]}]],[[{"lng":[-8090849.36022338,-8090847.64895794,-8090857.78966578,-8090859.50094768,-8090849.36022338],"lat":[5228039.04734146,5228023.39642026,5228022.2794819,5228037.93039943,5228039.04734146]}]],[[{"lng":[-8090850.47603237,-8090841.05743702,-8090843.53869116,-8090852.95728243,-8090850.47603237],"lat":[5227892.40797523,5227887.9622943,5227882.66687366,5227887.11254967,5227892.40797523]}]],[[{"lng":[-8090852.95728243,-8090845.94803723,-8090850.39926855,-8090857.4085083,-8090852.95728243],"lat":[5227887.11254967,5227883.80411192,5227874.30440782,5227877.61283901,5227887.11254967]}]],[[{"lng":[-8090814.13663207,-8090806.61129543,-8090809.16755755,-8090816.69288912,-8090814.13663207],"lat":[5227806.43067491,5227803.78091976,5227796.46772665,5227799.11747774,5227806.43067491]}]],[[{"lng":[-8090809.16755754,-8090819.53265252,-8090829.29579837,-8090818.93072661,-8090809.16755754],"lat":[5227796.46772665,5227769.74978188,5227773.56518565,5227800.28315175,5227796.46772665]}]],[[{"lng":[-8090860.13058715,-8090848.30075064,-8090853.78814069,-8090865.61796775,-8090860.13058715],"lat":[5227768.14630536,5227761.91419935,5227751.4214028,5227757.65349513,5227768.14630536]}]],[[{"lng":[-8090850.37261321,-8090838.70643941,-8090846.01822107,-8090857.68438183,-8090850.37261321],"lat":[5227757.59313206,5227751.62620807,5227737.2255355,5227743.19244153,5227757.59313206]}]],[[{"lng":[-8090847.5060609,-8090850.15405459,-8090856.51544174,-8090853.86745062,-8090847.5060609],"lat":[5227737.98652383,5227732.77124851,5227736.024914,5227741.24019288,5227737.98652383]}]],[[{"lng":[-8090822.12440817,-8090821.9307867,-8090844.66018766,-8090844.85384905,-8090822.12440817],"lat":[5227547.3223974,5227530.58151235,5227530.31666873,5227547.05755288,5227547.3223974]}]],[[{"lng":[-8090844.7385351,-8090848.77864387,-8090848.85845939,-8090844.81834769,-8090844.7385351],"lat":[5227537.08934549,5227537.04226429,5227543.94159022,5227543.98867149,5227537.08934549]}]],[[{"lng":[-8090848.85845939,-8090848.73186863,-8090859.90227595,-8090860.02887734,-8090848.85845939],"lat":[5227543.94159022,5227534.8803369,5227534.72312598,5227543.78437899,5227543.94159022]}]],[[{"lng":[-8090860.02887734,-8090860.06087205,-8090848.89045141,-8090848.85845939,-8090860.02887734],"lat":[5227543.78437899,5227546.07433823,5227546.23154954,5227543.94159022,5227543.78437899]}]],[[{"lng":[-8090822.0828609,-8090819.17672905,-8090818.99894692,-8090821.90507407,-8090822.0828609],"lat":[5227543.73014295,5227543.76400139,5227528.39220534,5227528.35834702,5227543.73014295]}]],[[{"lng":[-8090821.90507406,-8090831.03397237,-8090831.05968714,-8090821.9307867,-8090821.90507406],"lat":[5227528.35834702,5227528.25198341,5227530.47514871,5227530.58151236,5227528.35834702]}]],[[{"lng":[-8090972.49035324,-8090961.29211724,-8090968.2897108,-8090979.48790191,-8090972.49035324],"lat":[5227463.73120786,5227461.73094194,5227422.26775926,5227424.26800868,5227463.73120786]}]],[[{"lng":[-8090966.98572159,-8090963.30497376,-8090960.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---
## Projections checklist

- <i class="fas  fa-check-square "></i>: Are you using `leaflet`?
    - Project everything in EPSG 4326

- <i class="fas  fa-check-square "></i>: Are you using `ggplot2`? 
    - Choose a projection scheme that is appropriate for your data
    - Project everything in that scheme