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May 25 2011

Transect Grid Image Capture Pattern

We tried several methods of gathering ground data which ended up being unable to match up enough detail to project camera positions correctly in the software.  Borrowing a page from the flight capture patttern, I decided that what we needed was a large number of intersections between tracks, so that error-reducing loop closures would be maximized rather than rendering an area as one big track in which error accumulates.  A grid system accomplishes this simply and effectively.

The acquisition has several steps:

1) Secure the corners of the area, by marking out four points representing a 25 meter square, or quadrat, and collecting geospatial reference information on these corners.  Mark the corners using two things: First, a highly visible marker, and second, a barely visible marker that will be durable in the face of people attempting to destroy the site.

2) Interpolate points between these corners via the method of your choice, in order to construct 25 individual 5x5m grid cells.  Mark points on hte exterior with something highly visible, which will show up in a synth.  For interior points, use some unobstructive flag that is at least visible to the user.

3) Optionally, collect forest inventory information based on these 5x5m grid cells.

4) Pick a 'Home Point' and orient yourself pointing towards the center of the quadrat.  If the quadrat is part of a bigger area, pick a home point based on some consistent factor, like "the northernmost corner point".

5) Photography Stage 1. Based on that orientation, walk to your right between two columns of markers until you reach the end of the quadrat.  Come back between the next pair of columns, then walk back out in a switchback pattern until you reach the opposite end of the quadrat from the home point.

6) Photography Stage 2. Turn around and walk the exact same path, pointed in the opposite direction, until you reach the home point.

7) Photography Stage 3. This time, walk to your left between two rows of markers until you reach the end of the quadrat.  Come back between the next pair of rows, then walk back out in a switchback pattern until you reach the opposite end of the quadrat from the home point.

8) Photography Stage 4. Turn around and walk the exact same path, pointed in the opposite direction, until you reach the home point.

9) Run the images in Photoscan or other software with orthophoto-oriented optimizations turned off

10) Verify that the software has put cameras in the right place in the point cloud, and georeference the point clouds.


This pattern was tested in the GES485 field methods class (post coming).

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