The question '
What is a mosaic image?' was posted by one of the students ("emmagrace") in the
I'm a Scientist Event. Of course, I gave an
answer, but this one really needed pictures to go with it.
So, as a special feature, here's what a mosaic image is, along with a complete example of how we make some of the panorama photographs that feature on the KAIRA web log. This has been written especially for the students so, for the imaging scientist experts out there, bear with me while I explain it!
Mosaic ImagesOf course, as the other scientists answered on the website, a mosaic is indeed where an artist uses hundreds of tiny coloured tiles to make a picture. There are many splendid examples of mosaics both recent and from ancient times. A quick look at the
Wikipedia article will reveal some spectacular examples.
In a way, any digital photograph is a mosaic of thousands, if not millions, of tiny squares called
pixels. Some images, known as
photographic mosaics are composed such that each pixel is, in itself, an image.
However, there is another type of mosaic image. And this one is directly relevant to many scientific applications. The term is particularly used in astronomy and microscopy, mostly as this is where the technique is needed due to limitations of the optics of telescopes and microscopes. In these cases, a mosaic is a large image, made up several parts of and image. You can do the same thing with normal photography, in fact. These images are also called mosaics, as well as 'stiched', 'composite' or 'tiled' images.
Mosaic images like this are wonderful when the field of view of the scene is far larger than the field of view of the camera. Because the receiver array at KAIRA is so big, and the scenery so wide, we often used mosaicing to create our panorama images.
Here's how it works.
Let's imagine that you have a scene that you want to photograph... something like this. It is a wide open space, with some mountains in the background, clouds and dirt. It is where the KAIRA construction project is happening and we want a photograph of it all!
The problem is that our camera has a limited field of view. It can't see everything at once. You have to choose which bit you look at. For example, maybe just the middle.
If you take this photograph sure, it looks great, but we miss out on the edges and can't see the mountains.
So, what we do instead is take 3 photographs. They need to be slightly overlapping (and we'll see why later). But for now, you could simply line them up. Not bad. You get a rough idea, but it is still not too good.
Of course, you can cut-and-paste the images a bit, but there are other problems. The images are not necessarily flat (meaning, that they are a bit distorted at the edges). Also, they may have different exposures, meaning that one image may be slightly darker than another.
Take a look at the images now. They overlap nicely, but it is still very obvious where the line between them is.
To fix this, we compare the images with a special computer programme. What we do is mark matching points in one image, with the exact matching point in the other image. It might be a peak on the mountain or a light-coloured stone, or a part of the timber. It doesn't matter. The point is that we are telling the computer that this point on this image, matches that point on the other image.
Here's what it looks like, with the matching points flagged with little numbers.
If you then run the programme, it will work out the distortion of the individual images and correct for them. Look at the red and blue outlines of the centre and right-hand image. Although you may not have noticed the distortion in the original, it is actually quite pronounced and this prevents us from getting a good match.
Additionally, the software can adjust the brightness of each image, so that they match.
Finally, the image is stitched together and blended smoothly. If you didn't already know, you may not realise that this image is not one, but three images... all neatly mosaiced together.
And that is what the KAIRA build site looked like on 14th June 2011.
As I mentioned, we have used mosaicing for other images on our web log, such as
a view of Saana made from 17 images and
a view of the winter testing made from 50 images. And this technique is used in astronomy and geophysics for mapping, imaging and aligning huge sets of data.
PS: You can click on any of the images to see larger versions. Try it... especially on the last one!
However, here's a better one! This is a panorama, taken at approx. 23:45 on 23rd June 2011 (just after the mid-sommer). There is a hint of a rainbow, and the sun would drop out of the clouds, about 30-40 minutes after this sequence was taken. (Click on the image for the full-resolution version.)
Congratulations Onsala!
The KAIRA array has a few places where the signal cables will need to go underground. Because of the thawing and freezing, the cables can't be dug in directly, but must be ducted. However, the ducting lines can't be too long either, as it would otherwise either require huge tunnel and thus be too expensive.
While all the levelling work and surveying and all the other things have been going on at the main mound, from further south, there has been another KAIRA-related operation under way.
With the High-Band Array (HBA) field finally level, it is time to start work on the survey for the antennas themselves.
With the new reference mark, the corners were re-calculated with the correct azimuth. Then all the intervening grid points were calculated.
The final day of the 'I'm a Scientist – Get me out of here!' event. This is our last instalment on the KAIRA web log of this public outreach event. To all those IAS2011 participants and followers, thanks for visiting us and for keeping in touch during the event. Hopefully, you'll come back and visit the project and find out how we've been getting on with our radio receiver array construction project.
... TOM!
Although I've been evicted, some people might be interested in who actually did win the in the I'm a Scientist Get Me Out of Here event. And, as there are only two days to go anyway, I figured I'd put up those last to posts to keep everyone up-to-date with the final score. Today, the scientist voted off was Tim. Now only Sarah and Tom remain — congratulations to both of them for making it to the last day. The final is tomorrow!
How many times has the gravel lorry visited the site?
Of course, the main array itself is now complete, so these new loads that are coming in are for the storage, loading and turning areas at the southeast end of the array. These are essential for the building of the system and will also be used as a general staging area for the servicing of the antennas during the longer term operations phase.
For me, it has been a fantastic experience. It has also been a chance to look at my work, and the projects I've been involved in, in a different way. It has given me a chance to see things from a student's point of view again and, in a way, ask questions of my career, as if I was considering embarking on it all over again.
The RF-container needs a secure and stable location. This means that it has to be level, not subject to excessive snow drifts and allowing easy transfer of the signal cables from the array and into the input section of the 'container foyer'.
We've another photo that we forgot to include in our article the other day about the delivery of the power cable equipment. This is the cable reel, fully mounted on the spool sled, ready for running out.
It's been great working with Dalya. We are so sorry to see you go!
Around the site, there will need to be a protective fence. This is primarily for the reindeer. For one, we need to stop the reindeer from damaging the antenna array.
The cleared, flattened area just manages to stand out from the surrounding landscape. However, the stacks of timber HBA frames are very distinct.
As we saw in the first part, agricultural drains are being installed in the intermediate gravel layer on the KAIRA site.
The pipes themselves are 100mm in diameter. There are a number of them scattered across the array, and draining to the southwest, where the water will be distributed over the embankment (thus minimising the chance of erosion.
If they don't move, these little creatures are extremely difficult to see. However, just after I took this photograph, this one decided to make a break for it, scampered over a snow drift and disappeared under a layer of ice.
Here's a close-up photograph of the levelled gravel surface. It is very smooth and flat, but is easily disturbed at the moment by wheeled vehicles.
