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Georeferencing Retrolens imagery using Google Earth

· 6 min read
Duane Wilkins
Updated guidance

This post records a simple workflow that used Google Earth Pro desktop for visual image overlays. Google now treats Google Earth Pro as a legacy desktop application and will stop offering new downloads on 25 June 2027.

For new work, use the current historical imagery workflow. Use QGIS when the image needs proper georeferencing and a reusable spatial output. Use Google Earth in the browser when Google's imagery is useful for visual comparison. Use ArcGIS Earth as a free Windows desktop globe for local files and KML/KMZ, not as a replacement for a proper georeferencer.

Downloading a historical aerial photograph is only the first step. To compare it meaningfully with current information, the scan often needs to be connected back to place. That sounds like a technical problem, but georeferencing is also an act of interpretation. We are taking an image captured at one time, often for a survey purpose, and relating it to a present-day coordinate system and landscape that may have changed substantially since the photograph was taken.

For Māori GIS that distinction matters. The goal is not to make an old photograph submit neatly to the modern map. Awa may have moved, coastlines may have changed, roads may have been built and landforms altered. A photograph can help reconnect records and memories to whenua, but its position, its meaning and the conclusions drawn from it still need to be checked against other evidence and, where relevant, the people who know the place.

This older workflow used Google Earth Pro's Image Overlay tool to align an image visually. It can still be useful for people who already have Google Earth Pro installed, but it should now be treated as a legacy exploratory method, not the recommended way to build a new repeatable workflow.

What this method is good for​

A visual overlay can help with comparing broad landscape change over time, orienting an old aerial photograph during kōrero, identifying features that may deserve closer research, and creating a quick exploratory KMZ for an existing Google Earth Pro workflow. It is not the method to use when alignment accuracy matters, when you need a properly georeferenced raster, or when the result will become an ongoing GIS dataset.

The important point is that the overlay should support the kaupapa rather than become the kaupapa. A technically neat fit is useful only if it helps answer the question people are actually asking about the whenua.

The current preferred workflow​

Use Retrolens historical imagery workflows for the current guidance.

A sensible pattern is:

  1. Preserve the original downloaded image and its source information.
  2. Use QGIS Georeferencer when you need a spatially referenced raster that can be checked and reused.
  3. Use several defensible control points and record what you used.
  4. Keep the source date, image identifier and uncertainty with the output.
  5. Compare the result with other historical and current evidence rather than treating the image as a complete account of what happened to the whenua.

That final step is particularly important. An aerial photograph can show physical form, but it does not explain whakapapa, occupation, customary use, dispossession, drainage decisions, access changes or why a place matters. It should sit within a wider body of knowledge rather than above it.

Legacy Google Earth Pro method​

If you already have Google Earth Pro installed and only need a rough visual overlay:

  1. Open Google Earth Pro and navigate to the area.
  2. Choose Add → Image Overlay.
  3. Open the Retrolens image.
  4. Adjust transparency so the old image and current reference imagery can both be seen.
  5. Use several stable features such as long-established road intersections, river confluences, coastlines or major terrain features.
  6. Move, rotate and scale the overlay until the overall fit is useful for exploration.
  7. Save the result separately from the original source image.

Do not chase an apparently perfect edge fit. Historical aerial photographs can contain distortion, especially across terrain, and a visual overlay does not correct that distortion. If a river or coastline no longer sits where it once did, that mismatch may be historical information rather than a georeferencing error.

Accuracy and evidence​

A neatly aligned historical photograph can look more authoritative than it really is. Keep three things separate: what is visible in the photograph, how confidently the photograph has been positioned, and what you infer from the change between dates. Where a location is uncertain, keep the uncertainty visible. Do not let a GIS point or crisp overlay become stronger evidence than the source material supports.

Local knowledge should also be treated as an active part of checking the interpretation, not as a decorative anecdote added after the technical work. Somebody who knows the whenua may recognise a former channel, track, settlement area or other feature that the GIS operator would otherwise misread. The useful relationship is reciprocal: the image can prompt kōrero and the kōrero can change how the image is interpreted.

Māori GIS use​

Historical imagery may help reconnect records, memories and kōrero to place, but the photograph remains one form of evidence. It does not by itself explain land loss, occupation, use, whakapapa, naming or why a place matters. The georeferenced output should retain the source and the decisions used to position it so that somebody else can understand its lineage later.

Before sharing a georeferenced image, consider whether combining it with locally held information reveals places or relationships that were not public in the source photograph. Technical processing can create new information even when every individual input was already accessible. The people with authority over the local information should retain authority over that decision.

Where to go now​

The old Google Earth Pro method remains here because existing users may still find it useful. It is no longer the site's default recommendation for new work. Whatever tool is used, the useful discipline is the same: preserve the original evidence, make the transformation explainable, keep uncertainty visible and let the relationship with whenua and the kaupapa guide the interpretation.