Maps
Contour / isoline map
Joins geographic locations with the same value on a continuous surface.
- Map
- Geography
- Continuous surface
What it answers
Where are surface values equal?
- Shape
- Lines of equal surface value
- What it shows
- Gradients, peaks and equal-value boundaries.
Example
The question
Where are average house prices highest across the city?
Average house price, lines of equal value (contour map, SEK thousand per m²)
Illustrative data. It belongs to no company.
When to use it
Use it when
Elevation or other continuous spatial measurements.
Avoid it when
Sparse samples interpolated without a stated method.
How to read it
Read contour labels and spacing on a geographic surface.
Note
State interpolation when the surface is estimated.
The data you need
- Layout
- Geospatial grid or sampled points
- Fields
- X,Y: coordinates; Z: surface value; or a georeferenced raster
- One row is
- One sampled surface location or one raster cell. What is the grain?
- Rules
- State CRS, grid resolution and contour levels. Point samples need a stated interpolation method.
- Optional
- A proper gridded surface usually needs far more samples than this schematic example.
- Preparation
- Validate the input grain and missing values. State CRS, grid resolution and contour levels. Point samples need a stated interpolation method.
- Units
- State the mapped attribute unit, geographic level and boundary/coordinate reference. Rates need an explicit population or area denominator.
Worked example
Surface samples include 20 and 30; interpolation places a 25 contour between them under the chosen method. Contour geometry depends on the surface.
| X | Y | Z |
|---|---|---|
| 18.00 | 59.30 | 20 |
| 18.01 | 59.30 | 25 |
| 18.00 | 59.31 | 22 |
| 18.01 | 59.31 | 27 |