#BlackAndSTEM ...in GIS/Cartography Louise E. Jefferson: Mapping - - PowerPoint PPT Presentation

#BlackAndSTEM ...in GIS/Cartography

Louise E. Jefferson: Mapping Black History W.E.B. Du Bois / Whitney Battle-Baptiste, Britt Rusert: W.E.B. Du Bois’ Data Portraits: Visualizing Black America Gwendolyn Warren (also here): Detroit Geographical Expeditions Black Girls Mapp (also here): representation in GIS and election advocacy Anti-Eviction Mapping Project: participatory mapping of housing rights and more Vincent Brown: historian, uses interactive mapping to visualize slave revolts

Projections, why they matter, and districting

• 1. Introduction to projections
• 2. Why projections matter
• 3. Considerations for districting
• 4. Setting projections in QGIS

“Visually depicting space and power --- cartography --- is a political act in every sense. Maps represent things not as they are, but as we need or want them to be for particular purposes.

• - Jessica A. Krug, Fugitive Modernities

https://www.atlasob scura.com/articles/ mercator-peters-bo ston-map

From the Earth to a sphere to a map…

Earth to Sphere: Geographic Coordinate System (GCS)

(sometimes aka datum)

Geographic coordinates: Latitude and Longitude (parallels and meridians)

Examples of lat/long...

Boston: 42.4° N, 71.1° W Oahu, Hawai’i: 21.4° N, 158.0° W Montgomery, Alabama: 32.4° N, 86.3° W ⅓ and ⅔ cuts of New York State: 42° N, 44° N Tufts University: 42.40° N, 71.12° W MGGG’s Redistricting Lab at Tufts University: 42.4079° N, 71.1209° W

Thanks, XKCD

Sphere to (flat) map: Projected Coordinate Systems (PCS)

aka projections Note: projections include a GCS

Projected Coordinate Systems (PCS)

aka projections

Start with a developable surface

• Cylinder
• Cone
• Plane
• (mathematical)

All projections distort

https://www.leventhalmap.org/digital-exhibitions/ bending-lines/interactives/projection-face/

Projections also can preserve certain properties:

• Conformal: preserves angles (~shapes)
• Equivalent (equal area): preserves areas
• Equidistant: preserves distance to anywhere on

the map ○ …from either one point (planar) ○ …or two points (cylindrical and conic)

• Azimuthal: preserves direction from a single

point (all are planar)

• Compromise: balances shape, area, direction,

and distance

https://www.leventhalmap.org/digital-exhibitions/bending-li nes/interactives/tissots-indicatrix/

Winkel Tripel Robinson

Compromise Projections

Case: intersection with reference globe

Tangent: rests on top of the reference globe Secant: slices through reference globe Properties preserved at standard lines(s) In QGIS: Lat_1 and Lat_2

Aspect: orientation of developable surface to ref. globe

Normal: developable surface oriented parallel to Earth’s axis of rotation Transverse: developable surface oriented 90° from Earth’s axis of rotation Oblique: developable surface is neither normal nor transverse

Review

Pay attention to:

• Geographic Coordinate System (GCS)
• Projected Coordinate System (PCS, or “projection”)

○ Includes GCS ○ Centering ■ Standard lines: often lat_1 and lat_2 ■ Longitudinal center for normal case: lon_0

Why do projections matter?

Why do projections matter? In general...

• Getting your message across:

○ What does the world look like? Mercator versus Peters ○ Should North be up? ○

• Meeting the purpose of your map:

○ Navigation ○ Representing another variable in specific areas (e.g., race using color or dots) ○ Showing how places relate to one another ○

• Doing spatial analysis

○ Area of specific polygons? ○ Distance between places or from a point? ○

• Equal-area maps: more socially just?

...or maybe they don’t? “[W]e don’t need a new map; we need a new view of the world.” - Peters

Why do projections matter? In Districting/MGGG work...

Districtr Mapmaking

• Dot maps
• Choropleths

Compactness Precinct work: locally appropriate and consistent

Projections in QGIS

Coordinate Reference System (CRS):

• Geographic Coordinate System

(GCS)

• Projected Coordinate Systems (PCS,

includes GCS)

• Based on libraries used in open-source mapping

○ European Petroleum Search Group (EPSG) ○ Institut Geographic National de France (IGNF) ○ (Environmental Systems Research Institute: ESRI)

• EPSG numbers used as shorthand for projections

○ Can remember name or EPSG number ○ e.g., 3310: Albers Equal Area, Centered on California, using GCS NAD83

Projections in QGIS

Some common Geographic Coordinate Systems

Most common: ○ WGS84 (World Geodetic System 1984): EPSG 4326 ○ NAD83 (North American Datum 1983): EPSG 4269

Some Common Projections

(Projected Coordinate Systems)

• Albers Equal Area (conic)
• Lambert Conformal Conic (conic)
• Mercator (cylindrical)

○

UTM: Universal Transverse Mercator

○

Web Mercator

• State Plane: locally parameterized for each

state (two per state)

○ Lambert conformal conic ○ Transverse mercator

• Plate Carree (Cylindrical): “unprojected”

A note...

If a PCS is not set, QGIS has to guess or set one

○ Default: Plate Carree

Projections in QGIS

Identifying and Setting a layer’s projection

• Temporary: Layer Properties

→ Geometry and CRS

• Permanent: Export → Save

features as → CRS Note: If you attempt to bring in a layer with a different GCS, QGIS will ask to do a datum transformation to align the layers.

Setting a project’s projection

• Automatically set from first layer

added

• To change:

○ Layer: Set Project CRS from layer ○ Project → Properties → CRS

Guidelines

Choose what kind of distortion to minimize:

• Angles/shape
• Area
• Distance
• Direction
• More than one

Choose a projection that is EITHER:

• Locally parameterized, OR
• Generally appropriate and set local (custom) parameters (rare for

QGIS)

Clues your projection is “off”

• Known boundaries are at strange angles

○ Your projection isn’t centered on the area of your layer (e.g., a North America projection for Maine)

• When you zoom to a layer, you can’t see the others

○ Could also be an issue of generalization (level of detail)