Why are messages to space constructed as pictograms?

Question

The Arecibo message and both forms of the Dumas-Dutil message are constructed exclusively as pictograms. They are sequences of bits, grouped into "pages" which when viewed as black or white pixels, form low resolution images. These images express their meaning by way of pictograms. My question is why? OK, there are some concepts which require some form of diagramming, but for the first few pages of the Dumas-Dutil message, it's a set of equivalences. Couldn't they be more concisely and perhaps unambiguously expressed using an arbitrary binary code, rather than concocting an alphabet of glyphs to be low-res rasterized, laid out onto a binary canvas as the message, and then serialized into the form needed for transmission? What is the rationale behind the expression of the entire message in this form?

Assuming an alien technological civilization receives the signal, recognizes it to be artificial, and notices that it contains a stream of bits, are there not a whole host of further assumptions about what the message recipient would have to do, what choices they would have to make in order to interpret the message as intended? Can we be sure that they would even think to represent the bits in an array of pixels for further interpretation as images? What cues are built in to the message to facilitate this as the one and only thing to do at this step?

Addendum:

If the Dumas-Dutil message was presented to a human team of say, computer engineers with diverse specializations including image processing and cryptography, physicists, mathematicians, chemists, biologists, and astronomers, none of whom have any prior knowledge of the message, would they be able to correctly interpret it? Has this exercise ever been tried? This sort of thing was attempted with the Arecibo message with disappointing results.

Does the pictogram structure really improve the chances of successful interpretation, or do we have a requirement for the recipient to follow a sequence of steps, thinking like a human, each of which introduces the likelihood of an incorrect action and consequent failure to arrive at the intended result?

Answer 1

It would certainly be possible to construct a message that doesn't start with pictorial elements, but certain concepts are much easier to get across if you can "point to a picture". It's also easier to synthesize information and recognize patterns if you can view a lot of it at once in a structured way; organizing the data in multiple dimensions helps with that.

Since eventually most of these transmissions do want to include pictures, it's convenient to start with them, but it would be possible to just use something like ASCII encoding to do the initial mathematical education.

The first cue to facilitate interpretation as an image is traditionally the length of the sequence: for a single image, it will repeat after a number of symbols which is the product of two primes, say 127 x 131 = 16637. Assuming the recipient has a concept of mathematics comparable to ours, this will immediately suggest 2-dimensionality, and hopefully -- if the idea of a 2-D bitmap is familiar to them -- they'll guess it's an image. Just about any intelligence I can imagine with multiple senses and localized sense receptors ought to have some concept of a 2-D image, but clearly some conceivable intelligences will be left out.

Messages like the Dumas-Dutil can be worked out by humans without prior knowledge; it's been done several times (by single persons as well as teams). MJD blogged his decoding of the DD here (though I believe he started from the bitmaps and had some general high level knowledge of the contents). The Aricebo might be too brief and sketchy to really get much information across.

This Stack Exchange puzzle Q&A demonstrates the ability of a small group of people to work out such a message from the raw signal. More significantly, the signal is not a digitally coded bitmap, but a sort of vector display -- not a completely unknown representation to modern techies, but not the most familiar either.

Some years ago I also saw a similar puzzle on the internet; in this case the "aliens" started with a linear format, but their numeric and mathematical representations all had an interesting grammatical quirk to them; they eventually graduated to bitmap images, but given in a spiral hexagonal format instead of a rectangular raster grid. Their entire message was likewise decoded completely by a small team.