use a tilemap for esri imagery instead of tacking on blankTile=false

[jgravois]

greetings! 👋

currently OSM editors are requesting roughly 35 million Esri imagery tiles a month 🎉 and making roughly the same number of requests for tiles that don't exist 🚫.

this PR still needs work, but it introduces support for the 'tilemap' option i described in #4327 (comment)

the gist of it is that we can take advantage of a service that indicates whether or not tiles exist in a particular area of interest.

by asking whether tiles are present at zoom level 20 in the current extent of the map as soon as the edit session is instantiated, we'd forgo a lot of unnecessary requests for tiles.

[mmd-osm]

a bit of error handling might be good here... (url unavailable, cannot parse result, etc.)

[bhousel]

The bigger issue here is that we'll have to polyfill fetch / promises for IE11..
(But we need to do this anyway for D3 v5 / #4929)

[jgravois]

i don't have my heart set on using fetch, i just didn't know what you were using for generic requests to a CORS enabled web servers.

i've pushed up another commit which uses d3_json and aborts if an error is encountered. at this point i could use a hand triggering the function when editing is initially activated instead of waiting until the metadata context menu is opened.

[bhousel]

Cool! Thanks @jgravois - what is missing from this now? It probably would work if you switch the const/let back to old-school var.. (again strict ES5 for IE11/Phantom 🙄)

[jgravois]

switch the const/let back to old-school var

👍

what is missing from this now?

right now fetchTilemap() is only called when the background metadata window is opened w/ ⌘ + Shift + B.

i'm not sure how/where, but the method needs to be called as soon as users cross the zoom threshold to put iD into 'edit mode' (when Esri imagery is being used in the background anyway).

[jgravois]

• const > var
• found existing hook trigger requests to check the esri tilemap.

this is ready for review.

[bhousel]

Thanks @jgravois! I tried this out and ran into a few issues:

1. It looks like the tilemap url isn't put together correctly:

I also get a eslint warning around this code
309:13 warning 'x' is assigned a value but never used no-unused-vars

Would it make more sense to calculate z/x/y first and then call esri.url, and then append a /tilemap? Rather than calling esri.url first, then calculating x/y and using regex to replace parts of the url? (just guessing)

2. I think esri.fetchTilemap might need to use jsonpRequest instead of d3_json (similar to how the esri.getMetadata function works).
   I'm getting blocked by CORS.

[jgravois]

thanks for taking the time to check this out @bhousel!

you were definitely on the right track with the eslint warning. i made a silly mistake when i removed my template literal to please uglify after i finishing my own testing.

clarity.maptiles.arcgis.com definitely returns a CORS response header when the request is formed properly.

[bhousel]

Looks better now @jgravois - should there be code that puts the max zoom back to a higher number if the tileMap supports it? AFAICT it will just stay at [0,19] forever if it gets a negative result from the tilemap query..

[jgravois]

my thought was that once a user is zoomed in tight enough to trigger a tilemap request and encounters missing tiles it'd be appropriate to downsample from there on out.

that said, I didn't really consider the scenario where they zoom back out beyond 16 and move somewhere entirely different.

I'm AFK for a couple days but happy to take another look when I get back.

[bhousel]

my thought was that once a user is zoomed in tight enough to trigger a tilemap request and encounters missing tiles it'd be appropriate to downsample from there on out.

that said, I didn't really consider the scenario where they zoom back out beyond 16 and move somewhere entirely different.

Hmmm - I tested this by accident today by opening the iD walkthrough, which uses the Esri Clarity layer around Three Rivers MI, and it put the maxZoom to 19.

I'll see if I can find a way to have iD check the tilemap sometimes.

[bhousel]

I merged this 🎉
I ended up moving the code to call fetchTilemap into the main background() drawing function. But it will only do this if the user is already at > z18 and it will not fetch again unless the user moves more than 1km away. This way the user can move around to different places, and iD can adjust the Esri max zoom to 19 or 20 as needed.

[jgravois]

awesome @bhousel. thank you 👏!