Georeferencing a single image

[AM]

Hi everyone,

A client sent over a single nadir image taken with a drone and requested the equivalent of an orthomosaic essentially so that it could be pulled into arcmap to be analyzed/presented for a report with basemap features.

Is there a way to export a .tif version of a single image. The metadata has the elevation and relevant information that was taken with the drone, however, I'm unsure of how to process it in order to make it georeferenced.

Cheers.
AM

[Dave Martin]

A.M.,
I'm not really sure you can do this with PhotoScan, as by the sounds of it you're trying to reference and un-distort a single image?

What you can deliver depends on what information you have / can gather:

• If all you have is the image, the nadir position and frame orientation, I don't think you can do much of value.
• If all you have is the image, nadir position, orientation, and length of the sides of the image on the ground, you could hand-construct a .tfw - but the image would still potentially be distorted, unless it was from a planimetric camera.
• If you have the above plus a number of points on the image which you can the accurately locate on the ground - either existing known monumentation or recognisable points that you can survey - then you have a chance; the more points the better, and the flatter the terrain the better the results could be.
  

You really need a minimum of four points towards the four corners, more is better, especially if the terrain isn't flat, so you don't just have an affine but a higher-order correspondence across the image. Just like any other geo-referencing project, you should also keep a few known points back and then use them as check points to validate the transformed image.

I have done this with older single-shot aerial imagery by using Didger (by Golden Software), Global Mapper (by Blue Marble), and ArcMap; but many other GIS will do this with varying degrees of sophistication and accuracy, including, I believe, QGIS.

Commands and workflows such as geo-referencing and registering aerial images usually assume that you already have an un-distorted ortho image. The 'un-distorting' process is often referred to as "rubber sheeting" so you may find that a productive search term. ("Rectification" as a workflow in some of these systems can sometime refer more to applying a set of known lens corrections to an image).

I would suggest though that however you decide to un-distort and geo-reference the image (if you achieve it) that you endorse what you're delivering with a healthy disclaimer to manage the client's expectations and your liabilities!

Dave