global-crs-selection

name: global-crs-selection description: "Use when choosing a coordinate reference system for world-scale or near-global geospatial work, including global maps, world choropleths, antimeridian issues, polar coverage, global grids, or deciding between EPSG:4326, EPSG:3857, Equal Earth, LAEA, Robinson, or other world projections."

Global CRS Selection

Use this skill when the spatial extent is continental, global, transoceanic, or crosses the antimeridian and ordinary local projection advice is not enough.

Goals

• Choose a defensible CRS for world or near-world work.
• Match the projection to the task: storage, display, measurement, thematic mapping, or modelling.
• Avoid common global CRS failures such as Web Mercator misuse, antimeridian splits, and polar distortion.
• State tradeoffs clearly when no single projection preserves everything.

Core Facts

• EPSG:4326 is a geographic CRS in degrees. It is a good interchange format, but not a projected measurement surface.
• EPSG:3857 is for web mapping and visualisation. It is not appropriate for precision distance, area, or global statistical analysis.
• For world thematic maps, equal-area projections are usually the safest default.
• No single world CRS preserves area, shape, distance, and direction at the same time. The correct choice depends on the task.
• Polar and antimeridian-spanning data need explicit handling even when the CRS itself is valid.

Task-Based Selection

1. Data exchange or API storage

• Default to EPSG:4326 when you need broad interoperability.
• Keep in mind that coordinates are angular units in degrees.
• Do not compute area, buffer distance, or planar nearest-neighbour analysis directly in EPSG:4326 unless the tooling is explicitly geodesic.

2. Slippy maps and tiled web display

• Use EPSG:3857 only when the target is a standard web map stack.
• State clearly that the choice is for display compatibility, not analytical correctness.

3. World choropleths or global thematic maps

• Prefer an equal-area projection.
• Good defaults:
  • Equal Earth: good general-purpose world thematic map with preserved relative area.
  • Lambert Azimuthal Equal Area: strong choice for hemisphere-focused or pole-centred views.
  • Cylindrical Equal Area variants: useful when you need a simple global equal-area frame and accept shape distortion.

4. World reference maps for presentation

• Use a compromise projection when visual balance matters more than metric properties.
• Typical candidates: Robinson, Winkel Tripel, Natural Earth.
• Do not treat these as measurement-safe.

5. Global distance or area analysis

• Prefer geodesic calculations on the ellipsoid when possible.
• If a projected CRS is required, pick one that matches the region and metric of interest rather than forcing one world CRS onto all tasks.
• For hemisphere-scale analysis, consider a custom +proj=laea centred on the study area.

6. Polar work

• Avoid equatorial or generic world projections for Arctic and Antarctic analysis.
• Use a pole-centred azimuthal or stereographic projection appropriate to the task.

Common Country Defaults For Meters Or Feet

Use this section when the user is really asking for a practical projected CRS with linear units for local analysis, buffering, area, or engineering-style measurements.

Fast Rules For Units

• If the user says "I need meters," recommend a local projected CRS, usually a national grid or the correct UTM zone.
• If the user says "I need feet" in the United States, prefer the appropriate State Plane definition and confirm whether the workflow expects US survey feet or international feet.
• If the country spans several UTM zones, do not recommend one zone blindly unless the study area is confined to that zone.
• For countrywide statistical mapping, a national equal-area CRS may be better than UTM even when the user still wants meter-based units.
• If the task is legal, cadastral, or engineering work, prefer the CRS used by the authoritative local agency over a generic fallback.

What To Say When Unsure

• "For precise measurement, I need the actual study area within the country before I pick the best projected CRS."
• "For local work, use the relevant national grid or UTM zone; for countrywide mapping, use the country's standard national projection if one exists."
• "If the source data already comes from an authoritative local agency, keep its projected CRS unless there is a strong reason to transform it."

Workflow

1. Establish the spatial extent.
   • Single country, continent, hemisphere, global, or global with polar emphasis.
   • Note whether geometries cross the antimeridian.
2. Identify the real task.
   • Storage and exchange.
   • Tile display.
   • Thematic mapping.
   • Area or distance analysis.
   • Interpolation or modelling.
3. Check whether the current CRS is geographic or projected.
   • If EPSG:4326, confirm whether downstream operations are geodesic or planar.
   • If EPSG:3857, verify whether it was chosen only for web display.
4. Pick the projection family.
   • Equal-area for area comparison.
   • Compromise for presentation.
   • Azimuthal for hemisphere or pole-centred views.
   • Geographic only for storage, exchange, or geodesic workflows.
5. Handle edge cases.
   • Antimeridian crossing.
   • Polar extent.
   • Mixed CRS layers.
   • Axis-order confusion.
6. State the tradeoff explicitly.
   • What is preserved.
   • What is distorted.
   • Why that distortion is acceptable for this task.

Decision Rules

• If the user says "world map", ask whether the goal is visual presentation or quantitative comparison.
• If the user needs global area comparison, do not recommend EPSG:3857.
• If the user needs a broad, safe global default and no other constraint dominates, start with Equal Earth for thematic world maps.
• If the user is publishing to a standard web map, EPSG:3857 is acceptable for rendering only.
• If the workflow crosses the antimeridian, mention geometry splitting, wrapping, or longitude normalization before downstream overlay work.
• If the workflow is polar, do not leave the data in a generic world projection without justification.

Common Failure Modes

• Treating EPSG:4326 as if degrees were metres.
• Running buffer or area calculations in EPSG:3857 because the coordinates look metric.
• Choosing a world projection because it is familiar rather than because it matches the task.
• Ignoring the antimeridian, causing polygons or raster bounds to wrap incorrectly.
• Assuming one CRS can serve storage, visualisation, and measurement equally well.

Output Format

Spatial extent:
Current CRS:
Actual task:
Recommended CRS or projection family:
Why this choice fits:
What distortion or limitation remains:
Edge cases to handle:
Validation checks:

Guidelines

• Prefer named projection families over random EPSG guessing when working at global scale.
• Mention geodesic alternatives when the user asks for accurate world distances.
• Be explicit when a recommendation is a PROJ definition rather than a single EPSG code.
• Separate storage CRS recommendations from analysis CRS recommendations.
• For world maps, explain the tradeoff in plain language instead of assuming the user knows projection theory.

Example Triggers

• "Which CRS should I use for a world choropleth?"
• "I need a projection for global climate polygons that cross the antimeridian"
• "Should I use EPSG:4326 or EPSG:3857 for a world dataset?"
• "What is a good CRS for Arctic and Antarctic coverage in the same workflow?"
• "I need a global equal-area projection for analysis"