Scales FAQ

How accurate are the scales?

The scaling error of the photogrammetric alignment is usually much greater than the error stemming from the scales.

The accuracy of the target placement in our print files is better than 0.5 pixels for a 300 dpi file, i.e. better than 50 µm! This is true for all scales, be they 0.5 m long or 0.025 m long. 

The file is printed at 150 dpi, and the printer gives the error for a A0 sized print 84,1 x 118,9 cm) at less than 0.1 mm.

In effect, our experience with high resolution photogrammetry is that the accuracy of the scales allows errors of less than 0.0001 m consistently. In many cases, we have achieved average errors between scales of less than 0.00005 m.

What material are the scales made from?

Our scale bars are made from durable 5 mm thick PVC foam board. This makes the light-weight and robust, and guarantees that their corners are not tough enough to cut the fabric of backpacks or suitcases (as aluminum dibond might).

In the field, please weight down the scales with a rock or hammer, as anything not made from a really heavy material (e.g. metal) may be shifted around by the wind.

How do I use the scales during data capture?

In broad terms, you simply put the scales next to the object you wish to model in 3D, and make sure that your photographs show them in focus and without motion blur.

A detailed guideline for the efficient use of the Palaeo3D scale bars can be found here: LINK

Two-point, three-point - what's the difference? (How do I use three-point scales?)

A two-point (linear) scale has a target at each end, with the distance between targets defined. Such scales are suitable for scaling a model, i.e. for making sure it has the same size in the computer as it has in real life. Or that it is smaller or larger than in real life by a known factor, if that is needed.

The scales are simply placed alongside or on the object you wish to digitize. Later, the targets are define in the photogrammetry program, and their distance is entered to allow the program to scale the model accurately.

A three-point (planar) scale has three targets, which together form two linear scale bars that are at an exact right angle to each other. In addition to accurate scaling of the model, these scales allow preserving additional information with ease, e.g. the direction of strike and dip of a surface, or the orientation of your object in relation to the cardinal directions.

 

Detailed instructions on how to use two- and three-point scales are given in a PDF. Download it here.

How do I preserve cardinal directions or strike&dip?

In order to make use of the additional information a three-point scale can capture, the scale bars must be placed next to or on the object in a special orientation, usually level (use a small caravan level) or with one edge pointed due North (use a compass).

In the photogrammetry software, each target is then given world coordinates that match the distance between targets. For example, for preserving the cardinal directions of a model, a 0.25 m L-shaped scale can be placed with one arm pointing due North (as a consequence, the other arm will point due East or West). The target shared by the two linear scales is then given the world coordinates 0,0,0 m, and the other target of the scale pointing North is given the coordinates 0,0.25,0 m. This makes the computer model's Y-axis point due North, the X-axis point due East/West.

Detailed instructions on how to use two- and three-point scales are given in a PDF. Download it here.

Why are the scales double-sided?

Agisoft Photoscan Pro (company web page) can recognize the targets on the Palae3D scale bars automatically. This ability depends, among other factors, on the targets not being shown too large in the images. Therefore, our scales are printed double-sided, with one set of large targets on the front, and another set of smaller targets on the back. Depending on what you digitize, you can either use the front (for large objects, where the scales are shown from afar) or the back (for close-up photography). Or you can use some front-up and some back-up, to maximize the probability that Photoscan Pro can recognize the targets.

Detailed instructions on how to use the scales are given in a PDF. Download it here.

How do I use the scales to scale the model in Photoscan Pro?

In the Tools Menu of Photoscan Pro, select Markers, and then Detect Markers automatically.

In the dialog shown, select circular, 20 bit, and set the tolerance to 100, and click OK.

Once Photoscan Pro has detected the targets, open the Reference pane and select two markers that form a scale bar: Left-click one, press and hold CTRL, and left-click the other. Two-point scales always have a pair of consecutively named markers, starting with a even number. E.g.: target 260 and target 261. In three-point scale bars, the lowest number is the marker at the meeting of the two scales, e.g. target 2520 - target 2530 and target 2520 - target 2540.

Now, right-click and select Create Scale Bar.

Repeat this process for as many scale bars as you wish.

Once you have built all scale bars, enter their lengths in meters (e.g., 0.1) in the field Distance in the Scale Bar list in the Reference pane. You can also alter the accuracy (usually you should use 0.0001).

Now left-click the Update icon (two blue arrows forming a circle) at the top of the Reference pane to scale your model.
Photoscan Pro will now scale the mode and show yuo the individual and average error of your scale bars.

Detailed instructions on how to use two- and three-point scales are given in a PDF. Download it here.

.... and in Reality Capture?

Reality Capture is not able to automatically recognize targets. Therefore, simply place control points manually and create scale bars from them, as detailed in the Reality Capture tutorials and help files.

 

What's that extra white space for? (How do I place a scale vertically?)

Some of the scale bars have an empty space at one end, extending them beyond the target. That space is intended  as an anchor, in case you wish to place the scale bar vertically next to your object. Such a placement is often useful when using a turntable and tripod setup, e.g. during data capture on small objects with a macro lens.

If you use Ethafoam or Styprofoam as a support material, simply cut a small gash into it and stick the empty end of the scale into it.

Detailed instructions on how to use two- and three-point scales are given in a PDF. Download it here.

Can you make custom scales for me? With my logo on them, or for use on a turntable?

Yes, we can!

Depending on what you need, it can be a simply (and thus cheap) change to the print file, or a major re-design. Get in touch, and we can give you a quote!

What makes these scales so user-friendly?

Our scales are very easy to use, because they have been designed to work well with the typical user workflows in common photogrammetry programs such as Agisoft Photoscan Pro or Reality Capture.

In Agisoft, the targets on the scales can be detected automatically, saving the user the time and bother of manually placing markers. However, even if you have to place markers by hand, e.g. in Reality Capture, the scales make this task easy! Their markers are numbered in large, bold letters, so that the target number can be read easily even in small image previews. Even in slightly blurry photographs, the numbers are easy to read.

Similarly, the scale bar length is written on the scales in large numbers. When setting scale bar length in your program, you can simply glance quickly at the image preview and find scale length, without the need to open photographs and hunt around for the required data.

And even better: the target numbers encode the scale bar length!
- three-digit target names starting with 1 (e.g., target 122) --> 0.1  scale
- three-digit target names starting with 2 (e.g., target 261) --> 0.25  scale
- three-digit target names starting with 4 (e.g. target 406) --> 0.4 m scale
- three-digit target names starting with 5 (e.g. target 504) --> 0.5 m scale
- four-digit target names starting with 1 (e.g., target 1570) --> 0.1 m L-shaped  scale
- four-digit target names starting with 2 (e.g., target 2770) --> 0.25 m L-shaped  scale.
Within a two-point scale, the lower number target is always an even number. Thus, targets 110 and 111 form a scale bar, as do 112 and 113, not 111 and 112.
In L-shaped scales, the lowest target number is always the target that is shared by the two scales: e.g., 2750 - 2760 and 2750 - 2770.
As these rules are applied across all scales we offer, you can build in-program constraints just from the target names quickly!

But ease of use already starts during data capture with our scales!
For example, each scale has a color field printed on it. A quick glance at the colors is thus enough to ensure that you do not accidentally use the same scale twice in one project.

Also, the scales are light-weight and made from a soft enough material that they will not scratch things. This means that they are ease to carry around and easy to place - even right ON the object you wish to digitize. If you need to tie down a scale, for example to a mounted dinosaur skeleton, you can easily do so: a screwdriver is sufficient to drill two holes through a scale, which can be used with a bit of string or wire to tie a scale down.