Frequently Asked Questions

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Mensor FAQ

What does Mensor do?

Mensor employs area correlation stereo matching techniques to reconstruct a 3D representation from stereo 2D images. It creates depth maps from stereo pairs of images, e.g., satellite images, aerial photographs etc.

It can also detect change, i.e., it can show which pieces of a pair of images are similar and which are different.

What is a stereo pair?

A stereo pair is two photographs of the same scene taken from slightly different angles or positions. Mensor uses this information to re-construct the scene in 3D.

What can it be used for?

Producing bespoke topographic maps and digital terrain models from two or more images. This can be done at various scales, but it works best with small scale imagery, e.g., satellite imagery and aerial photography. The system could be of use to archaeologists, ecologists and others wanting to produce a digital terrain model of a site cheaply (using LIDAR/RADAR can be expensive, images for Mensor could be acquired by a model helicopter or blimp).

Old imagery can also be used; for example, it is possible to show the lie of the land in the 1920's or 1930's provided multiple photographs of the same scene exist.

Change detection – showing which parts of a pair of images are the same and which are different. For example, it can be used to analyse change in satellite images, e.g., growth of urban areas, logging of rain forests etc. In principle, any pair of contiguous images can be matched; for example, thematic mapper imagery can be compared with aerial photography. Mensor can also be used to track growth of warts and moles or to identify fingerprints.

Is it straight forward to use?

Yes, the system is essentially automatic. It reads in a pair of stereo images and produces a digital terrain model or change map (depending on requirements).

Does it need anything else or will it work “out of the box”?

For change detection or monitoring optical flow, it will work “out of the box”. If you want to generate a Digital Terrain Model, you will need to provide a camera model. When used for topographic mapping, Mensor has inbuilt procedures to output map information (position & height) in a variety of map projection formats, e.g., local vertical, Lambert Conformal Conic, UK National Grid, Universal Transverse Mercator, Geodetic (latitude, longitude, height) etc.

What is a camera model?

It is a computer program which describes the geometry of the camera that was used to acquire the images.

How do I get hold of a camera model?

Mensor comes bundled with a camera model for simple central perspective cameras (like digital cameras and webcams). It also contains a few other camera models for satellite sensors, e.g., SPOT, ASTER. If anything else is required, we can produce the model for you. We would discuss your specific requirements with you directly.

What does it run on?

Mensor was developed on Linux/UNIX systems. It can run on these systems (and also MAC OSX). It can also run on Windows in the Cygwin environment.

What is the minimum computer specification required to run Mensor?

Mensor can run on systems with several hundred megabytes of available disk and a minimum 256MBytes of memory.

What sort of timescales are involved?

It is able to generate change detection maps and digital terrain models from large images (6000 pixels x 6000 pixels) in a couple of minutes when run on modern PC hardware.

What input formats can it use?

The system can use images in JPEG, TIF. GIF, HIPS, and PBM formats.

What map projections does it offer?

When generating digital maps they can be output in a number of map projections including UK National Grid, Lambert Conformal Conic (LCC, “French Lambert”), Universal Transverse Mercator (UTM) and geodetic (latitude, longitude, height) – filters for other map projections can be written on request.

What output formats can it produce?

Several formats are available. Change maps and digital terrain maps can be output as images. Digital terrain models can also be output as a (long) list of (x,y,z) that is position and height co-ordinates in a map projection selected by the user.

Does the system have sensible defaults?

We think so. For example, topographic map output defaults to Universal Transverse Mercator; Change output defaults to change image.