[Talk-us-massachusetts] aerial comparisons

[Greg Troxel]

hobbit at techno-fandom.org writes:

> I've been flipping through various image layers when trying to find a small
> reference detail, such as visible parts of trails and stone walls in winter
> shots, etc.  It's also interesting to see how some of the park boundaries line
> up [well, almost in some cases] with the parcel layer.  Some GIS-years are
> clearer than others and it's not any preferred one for everything.

Beware that the parcel layer is highly variable.  From measuring actual
corner monuments (reading deeds, finding iron rods, etc.) with RTK,
sometimes the parcel layer is 30cm off.  Sometimes it is multiple
meters.  Sometimes, but not often, it has geometry (shape) errors, not
just registration errors.  But on balance it is pretty good and better
than you will do with non-RTK, maybe not as good as you can do with
MassGIS 2019/2021 imagery if the stone wall is the boundary and you can
see it.  Beware also that other imagery is not necessarily georeferenced
correctly.  The MassGIS layers have a published spec of something like
0.1 to 0.15 cm RMS in each of easting and northing, and they have done
post-production QA with check points.  But, with variable height, this
is tricky as most of the imagery is oblique and height errors cause
horizontal errors.  They try to correct for this.

> What I find super-interesting, and this is probably my naivete showing, is
> how the GIS and Strava and whatever layers are accessed through a template
> URL that dynamically gets filled in with the right magic numbers.  I guess
> there's a standard for how all that is supposed to work?  How similar is it
> to the OSM tiling?

It is called Tile Map Service and the basic idea is:

  spherical mercator projection
  zoom levels 1 to N, N usually at most 22
  standard scale per zoom level
  tiles 256x256 (I'm 99% sure), standard numbering
  URLs often similar, but x/y/z substituted in

This originated early on, I think google maps, and now pretty much all
raster maps for the normie web are served in this format.

There are two js libraries for displaying maps and while they do a lot,
TMS is the standard approach.  They are leaflet, the current mainstream
choice, and openlayers, the mainstream choice 10 years ago, still in use
somewhat.

There are also Web Map Services which are more hard-core GIS with
different projections, and people use those in qgis and ArcGIS.  And Web
Map Tile Services.  I basically use TMS and downloaded .jp2 tiles of the
MassGIS imagery, plus just .tif of old maps.

For pointers to read, this is a reasonable starting point:
  https://en.wikipedia.org/wiki/Tile_Map_Service
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 194 bytes
Desc: not available
URL: <http://lists.openstreetmap.org/pipermail/talk-us-massachusetts/attachments/20220722/ff9dcfa3/attachment-0001.sig>