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Compare historical and modern geography

Research question: How has a selected landscape feature changed between an older source and the present day, and how much of the apparent difference could be source or alignment error?

Useful for: taiao teams, historical researchers, whenua projects, planners, restoration teams and GIS practitioners investigating landscape change.

What you will produce​

  • a source register for historical and current imagery/maps
  • separate historical and current feature geometry
  • a documented change-measurement table
  • a two-date comparison map
  • an alignment/error note explaining how source quality affects interpretation

Working structure​

Create two project tables rather than downloading synthetic templates:

TableUse
source_registerRecord source date, reference, georeferencing or alignment information and provenance.
change_measurementsRecord measured change separately from any explanation of its cause.

Possible subjects include river channels, wetlands, shoreline, roads/tracks, settlement pattern, forest extent, gravel bars or restoration areas.

Choose one main change question for the exercise. A focused comparison is easier to verify than digitising everything visible.

Source and checking​

For each historical and current source, record provider, source reference, capture/publication date, scale or resolution where known, licence/attribution, alignment status and important processing. Use Provenance and source checking.

Keep measured change separate from causal explanation. The GIS can show that geometry differs between two dated sources; explaining why usually requires additional evidence.

Step 1: choose the older source​

Use Retrolens for historical aerial photography where available. For a scanned map or survey plan, use Georeferencing historical maps.

Record:

  • source organisation
  • image/map reference
  • capture/publication date
  • scale/resolution where known
  • licence/attribution
  • whether the image is already orthorectified/georeferenced

Step 2: keep the source unchanged​

Store the original file in 01_source. If georeferencing is required, save the transformed raster to 02_working rather than overwriting the source.

Record the source in source_register, including its date, reference and any alignment information.

Step 3: align only when required​

For a scanned map, select stable control points that existed at both dates. Road intersections, survey control or durable building corners can work. Avoid using a riverbank as a control point if river movement is what you are measuring.

Record the transformation, control points and residuals.

A good overall RMS/residual value does not guarantee equal accuracy everywhere. Check independent locations across the map.

Step 4: add current reference data​

Add current imagery from a known capture date. Avoid a web basemap with unknown mixed capture dates when the analysis depends on year-to-year comparison.

Add contextual data only if it helps answer the question: parcels, river network, contours or place names.

Step 5: digitise historical and current features separately​

Create two layers or a single layer with a reliable source_date field. For many teaching projects, separate layers are clearer:

  • feature_historical
  • feature_current

Use fields:

feature_id
source_id
source_date
interpretation
confidence
notes

If the feature edge is fuzzy, do not digitise with false precision simply because you can zoom further in.

Step 6: measure change appropriately​

River or shoreline​

Compare channel or shoreline positions at selected transects, or calculate areas between interpreted lines/polygons. Record the method.

Wetland or vegetation extent​

Calculate polygon area in EPSG:2193 and report both source dates. Consider classification ambiguity at the edges.

Buildings or roads​

Count or compare mapped features, but distinguish demolition/new construction from features simply obscured or unclear in one image.

Step 7: test for alignment error​

Before interpreting a 5 m shift as real change, inspect stable features nearby. If every road corner is displaced 5 m in the same direction, the problem may be image alignment.

Record an alignment_note in change_measurements so the interpretation remains connected to source quality.

Step 8: create two outputs​

Two-panel print map​

Often the most citable format. Use the same extent and scale for both panels and label dates prominently.

Swipe or interactive comparison​

Useful for exploration and public communication where the web platform supports it. Still include source dates and attribution outside the swipe control.

Quality checks​

  • Older and current source dates are explicit.
  • Historical and current geometry remain separate.
  • Georeferencing residuals/settings are retained where applicable.
  • Measurements use a projected CRS suitable for metres/area.
  • Apparent change has been checked against stable features.
  • Interpretation fields distinguish visible evidence from inference.
  • Output includes source references.

What the result means​

The project can show a mapped change between two source dates and quantify that change using a documented method.

It does not automatically explain why the change occurred. Attribution to drainage, engineering, climate, forestry, restoration or another cause requires additional evidence.

Continue with Historical aerial imagery, Imagery change through time, Georeferencing historical maps and Cartography.

Last reviewed: 4 September 2026

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