Some background: web developer, with some experience of Google Maps, currently developing a fairly complicated tool for archeological studies. Outside of Google Maps, I have little to no knowledge of mapping services, gis and whatnot.

I have been looking into OpenData sets to implement, and one which I am particularly interested in raised several questions.

The data is at http://www.datiopen.it/it/opendata/Regione_Lazio_Carta_geologica_scala_1_25000 , tab "Mappa".

My first reaction was, geological data, just the thing, awesome.
Then I saw it runs up to about 16,000 elements, all polys - less awesome, but not yet necessarily fatal.
I finally downloaded the data, and found it is a huge shapefile (35Mb zipped - 55Mb unzipped). I learned about gdal just about enough to convert the shapefile to an even worse 112Mb kml.

Even stripping away all the extended data from the kml, or picturing it conveniently stored in a database table, it does not look like any amount of data you would wish to serve over an internet connection.
The linked page, however, does it in but a few seconds.
The way it reacts to mouse events makes me think it simply finds out which poly the clicked coordinates fall within, but, here's what puzzles me, at the same time the contours of the clickable areas are already visible on the map before clicking - suggesting the polys are already loaded.

As this is making no sense to me, I suspect Something Everyone Already Knows But I Don't is at work here, possibly starting with the environment. I have spent some time reading forums and SE itself, but lack of experience leads to lack of perspective.

Can anyone tell by looks what is happening on the linked page, and tell me where to direct my research?

  • I clicked Mappa tab and for more than 2 minutes it's been displaying "Creazione dellamappa in corso". Can you give more information about use case you try to create? Are you trying to display all of the data as soon as map opens? Or are you trying to show data based on filters or by clicking on the map? – senol ozgur Jul 28 '17 at 8:12
  • That's odd, it always takes 3 seconds tops for me. My end purpose is to show vast amounts of data as smoothly as possible, of course. But in this specific case, I am looking for perspective into what kind of environment and approach the linked site might use. Alas, since posting the question I have learned some more about gis and I am afraid the answer is beyond our capabilities as a no profit. – deg Jul 28 '17 at 13:54

What is happening in all probability is the overlay is served as one would expect with a tileserver. Clicking anywhere interrogates a (presumably) spatial db about the clicked coordinates and returns the data in whatever way, JSON would do fine.

Which is why every click brings up an ajax interaction rather than, say, already know what was clicked.

(Self answered after a month of learning more on the subject.)

Edit: and after actually putting it into practice, I can add that there is no need for the dumb reload they do every time you pan the map.

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