Spike Map
A map using vertical spikes whose height encodes data magnitude at each location — taller spikes mean larger values, creating a dramatic skyline effect that reveals geographic hot-spots instantly.
// 01 — The chart
What it looks like
A spike map showing city populations. Taller spikes represent larger populations, creating a dramatic skyline that highlights major metropolitan areas.
// 02 — Definition
What is a spike map?
A spike map is a thematic map that places vertical spikes — narrow triangles or thin lines — at geographic locations, with each spike’s height proportional to the data value it represents. The result resembles a city skyline or a bed of nails rising from the map surface.
Unlike proportional symbol maps that use area (circles), spike maps use height as the visual channel. This is a deliberate advantage: humans perceive length more accurately than area, making spike maps easier to compare across locations. A spike twice as tall unambiguously represents twice the value.
Spike maps gained widespread attention during the COVID-19 pandemic, when media outlets used them to show case counts surging in specific cities. Their dramatic visual impact — tall spikes erupting from the map — makes them particularly effective for communicating urgency and geographic concentration.
Key advantage: Because spike maps use height (a linear channel) rather than area (a squared channel), they avoid the perceptual distortion common in bubble maps. A spike that looks “twice as tall” actually is twice the value — no mental math required.
// 03 — Anatomy
Parts of a spike map
// 04 — Usage
When to use it — and when not to
- You want to show magnitude at point locations on a map
- Accurate comparison between locations matters — height is easier to judge than area
- Your data has a few very large values and many small ones (high skew)
- You want a dramatic, attention-grabbing visualization for communication
- Bubble maps would create overlapping circles in dense regions
- Your audience should see both geographic position and relative value simultaneously
- Your data is area-based (rates per region) rather than point-based — use a choropleth
- Spikes would overlap and obscure each other in dense areas
- You need to show multiple variables per location — spikes encode only one value
- Your audience needs precise numeric values — spike heights are hard to read exactly
- All data values are similar — spikes will all look the same height
- The map is small or low-resolution — thin spikes become invisible
// 05 — Reading guide
How to read a spike map
Follow these steps whenever you encounter a spike map.
Read the title and understand what the spikes represent
Is each spike showing population, COVID cases, revenue, or something else? Understanding the variable is essential before interpreting any heights.
Check the scale or legend
Look for a height legend or labeled reference spike. Without knowing what a given height means numerically, you can only compare spikes relative to each other.
Find the tallest spikes first
These are your extremes — the locations with the highest values. Tall spikes naturally draw the eye, which is exactly the point of this chart type.
Look for geographic clusters of tall spikes
Do tall spikes cluster along coastlines, around capital cities, or within certain regions? Spatial clustering often reveals underlying factors like urbanization or infrastructure.
Compare spike heights, not widths
Spike width is typically fixed and carries no meaning. All information is encoded in height. A slightly wider spike does not mean a higher value.
// 06 — Pitfalls
Common mistakes
×Spikes too wide, obscuring the map underneath
Fix: Keep spikes narrow — 1–3 pixels wide is standard. The power of spike maps comes from their minimal footprint on the map surface.
×No height legend or reference scale
Fix: Always include a labeled reference spike or numeric scale so readers can translate visual height into actual values.
×Overlapping spikes in dense areas
Fix: Use semi-transparent fills so overlapping spikes remain visible. Alternatively, aggregate nearby points or use a tilted perspective view.
×Using spike maps for area-based data
Fix: Spike maps work for point data (cities, stations, events). For data tied to regions (rates per state), use a choropleth or cartogram instead.
×Inconsistent spike direction or baseline
Fix: All spikes should grow upward from the same baseline. Mixing directions or using a curved baseline makes comparison impossible.
// 07 — In the wild
Real-world examples
COVID-19 case counts by city
Major news outlets used spike maps during the pandemic to show surging case numbers. The tall spikes erupting from New York, London, and Mumbai created a visceral sense of the crisis that no bar chart could match.
Global population distribution
The Observable and D3.js community popularized spike maps showing population by city. The skyline-like visualization instantly reveals that most of humanity is concentrated in a few megacity clusters.
Earthquake magnitude mapping
Seismologists use spike maps to show earthquake intensity at recording stations. Tall spikes radiating from an epicenter make the geographic attenuation pattern immediately visible.
// 08 — Quick reference
Key facts
// 09 — Variations
Types of spike maps
The basic spike concept has several visual variations depending on the rendering approach and data complexity.
Triangle spike
The classic form — narrow isosceles triangles rising from the map. The filled shape makes them visible at small sizes.
Line spike (needle)
Uses simple vertical lines instead of triangles. Minimalist and clean, but harder to see at small scales.
Color-coded spike
Spike color encodes a second variable (e.g., category or change direction) while height encodes magnitude.
3D extruded column
Rectangular columns extruded from the map surface. Often used in WebGL-based tools like deck.gl for interactive 3D views.
// 10 — FAQs
Frequently asked questions
What is a spike map?+
A spike map is a thematic map that places vertical spikes — narrow triangles or thin lines — at geographic locations, with each spike's height proportional to the data value it represents. The result resembles a city skyline or a bed of nails rising from the map surface.
When should you use a spike map?+
Use a spike map when you want to show magnitude at point locations on a map. It also works well when accurate comparison between locations matters — height is easier to judge than area, and when your data has a few very large values and many small ones (high skew).
When should you avoid a spike map?+
Avoid a spike map when your data is area-based (rates per region) rather than point-based — use a choropleth. It is also a poor fit when spikes would overlap and obscure each other in dense areas, or when you need to show multiple variables per location — spikes encode only one value.
Is a spike map suitable for dashboards?+
Yes — a spike map can work well in dashboards as long as the panel is large enough for readers to perceive the encoded values, has a clear title, and includes the legend or axis labels needed to interpret it.
What category of chart is a spike map?+
Spike Map belongs to the Geospatial family of charts. Charts in that family are designed to answer the same kind of question, so they often work as alternatives when one doesn't quite fit your data.
How do you read a spike map?+
Start with the axis labels and legend, then look at the overall shape before zooming into individual marks. Compare prominent features against the rest of the data, and verify any conclusion against the underlying numbers when precision matters.