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  LiDAR News     

Targets for Traveling Print E-mail
Written by Thad Wester   
Thursday, 21 February 2013

Traveling and transporting equipment can be a cumbersome and exhausting task - if you let it control you.  Scanning a job 2,500 miles away?  Let’s see, you’ll need the scanner(s), batteries, chargers, laptop(s), targets, tripod(s)... you get the picture; and if you’ve done field work you understand the hassle and worry that comes along with moving $150,000 worth of relatively heavy equipment.

Scanning a last minute job, a 3 hour flight away?  Fed-ex can ship your gear over night, but if your shipping all the gear listed above, expect to pay several thousand dollars. And sure that is a billable expense, but it still adds to the cost of the project. Traveling and transporting equipment efficiently is a weekly struggle for anyone doing fieldwork with laser scanners.

When I began my laser scanning career at East Carolina University my typical traveling gear was a scanner (Leica HDS C10) + tripod + ruggedized laptop + 4-5 HDS single pole tripod targets stuffed into a hard golf club case.  The single pole targets are overkill for most jobs, but I started out scanning jobs that required them (long distances over rugged terrain). And to be fair, the HDS single pole targets were and still are worth the extra hassle and expense in that type of environment.   (As an aside, we scanned in some very rugged environments - a memorable location is documented here.) 

Some were so rough and remote that you were more likely to be hit by falling rocks then have cell service. On more than one occasion, someone would have to grab the scanner and seek shelter mid-scan as rockfall rained down on our scan position. We packed in our Leica ScanStation 2 (and then later on our two C10’s) into many very steep and rugged areas in both southern California and the front range of the Colorado Rockies.

So now that I am scanning in less complicated settings (buildings, homes, gun-ranges, etc...) losing the weight and expense of those single pole targets was something I looked forward to. But, I wanted to gain more than just reducing my shipping costs.  I wanted my new traveling targets to be:

- Cheap enough to be expendable (~$20)

- Rugged

- Reliable as a target

- Small / Lightweight

I know of two options that meet my requirements; black and white printable targets from Leica (less rugged) and homemade spherical targets made from light globes (more rugged).  So I set-out to build a set of spherical targets, and also to improve the paper targets, while being mindful of my four requirements.

Leica Black and White Printable Target

Being made of paper is great for transportation, but not so great when you set it up as a target.  In urban areas, taping the targets to flat, unmovable surfaces is easy enough. In other environments, you must improvise.  I’ve found that individual bathroom tiles work great for this.  Home Depot sells bathroom tile for less than a dollar a square, meaning that you can afford to buy it on location, and throw it away (along with the paper target) when you’re done.  To make this work, duct-tape the paper target tightly to the tile. Now this target isn’t exactly bullet proof (i.e. it moves easily), so you’ll want some extra redundancy in your control network, but I have used them to great effect on a number of scanning projects.

Magnetic Spherical Target

From my perspective a homemade magnetic spherical target has two advantages over a paper target; it can be acquired from any direction AND it can be easily mounted securely above the ground, given you have a metal structure that has height. Here is the recipe:

Materials (per target)

- 6 inch lighting globe

- Grey primer spray paint

- White matt finish spray paint

- 4 inch shower drain

- Gorilla glue

- (2) 1.5” rectangular mounting magnets

- (2) Flat slotted screws with nuts

Method

Paint your lighting globes.  One coat of primer and one coat of white.  While the paint dries, remove the metal bottom of the drain and bolt the two magnets onto the metal grate ( see image).  Reattach the metal bottom to the drain.  Once painting is complete, glue the globe to the top of the shower drain, using plenty of glue.

Field Use

After using the two types of targets over the past few months, I have some basic, semi-empirical data to judge them by.  The spherical targets are definitely my go to targets, mainly because they require fewer points for an accurate target acquisition.  If you're using a C10, this isn’t as much of an issue, but if you're using a HDS7000, which cannot acquire targets, it’s nice to get by with a few less points on your targets if your point spacing is less than planned.  One thing to note is that it isn’t a good idea to set your spherical targets on non magnetic surfaces.  They were not designed for that, and unless they are weighted, are very light and WILL move.

Although not a scientific exercise, I have experienced similar target error with both paper and spherical targets. In the field I consistently obtain errors of 0-1mm below 50m; 1-2mm between 50-100m; and 2-4mm above 100m.

###

Thad Wester is the founder of Clarity Scanning.  Look for him at Spar International 2013, where he will be giving a presentation on 3D printing from Point Clouds.


 
 
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