OpenGeofiction:Topo layer

What is a topo layer?

That's easy! The term "topo" (short for topographic) refers to a style of map that shows the physical elevation of land, either through shading or drawing contour lines or both. There are many topo renders available for the OpenStreetMap platform. These include Tracestrack Topo, CycleOSM and the venerable OpenTopoMap.

How is a topo layer made?

All the various implementations of topo style for OpenStreetMap rely on the easy availability of free data from the United States' NASA. This agency did a planet-wide survey of the elevation of all the land in the world on an arc-second (approx. 30-meter) grid, using radar, and published that data, online, for anyone to use. This data is called "SRTM". Developers implementing OpenStreetMap's various topo layers are able to download and use this SRTM data to use as a raw source of the detailed topographic information needed to draw the map. So, in the case of OpenTopoMap, the SRTM data is loaded to the render machine, processed in various ways, and turned into the hillshading, contour lines, and map-elevation color shading that we all know and love.

Does OpenGeofiction have a topo layer?

Yes it does! But it's not a simple thing to have or do.

Think about it: as explained above, for the real world, getting topo data is matter of launching a spacecraft or satellite with a little radar range-finder on it, and flying over the surface of the earth finding how high each part of the earth is relative to some baseline (i.e. mean high sea level). Easy! But OpenGeofiction is on a fictional planet. To repurprose Virginia Woolf's famous line: There's no there, there.

Instead we have to sit down and make up each square of elevation information - there's got to be a 30-meter square grid for our entire imaginary planet, and we have to invent the elevation for each and every square! And just randomizing this isn't going to work - real world topographic data betrays the evolved geophysical patterns of our planet, influenced by everything from plate tectonics to vulcanism to erosion to human landscaping. The data is going to have to either be "drawn" by creative human hands, or "drawn" by well-trained machine-learning AI's. Both are possible in principle, but then we have the data problem.

The data problem is this: the NASA SRTM format was developed for storing real world data. But no one ever thought: we need a nice, user-friendly, SRTM file editor! Why? Because no one needs to edit real-world data. It came from a super-precise scientific gadget, who's going to need to edit that? They could only mess it up.

So there simply does not exist any easy way to get the "imaginary" SRTM files that we need for our topo layer. Some geofiction genius is going to need to write a converter.

Luckily, OpenGeofiction had such a genius. The original creator of OpenGeofiction, Thilo, wrote a little program that turns a standard OSM XML format file (.osm) that has been filled with hand-drawn contour lines into a pseudo-SRTM format file (.hgt). Then we just need to install the standard OpenTopoMap software on our render server, copy the pseudo-SRTM files across, and voila, topo for OGF! All that custom code has recently been brought up to date - good news!

Still, we have a whole planet to cover. At this point in time (the time of writing this article, early 2022) the overall coverage for OGF's topo layer is tiny, and of fairly low quality, on average, so far.

Where can I see OGF's topo layer, right now?

For ease of administration and maintenance, the "active" areas of the topo layer for OpenGeofiction are divided into "zones." This map shows the active zones and squares within them. You can also toggle on/off inactive zones, blank squares...

Loading map...

You can view on the OpenGeofiction map using the CyclOGF or Tracestrack Topo layers.

What are the technical specifications of the OGF topo layer?

Tracestrack Topo

You can find the tiles at this path (e.g. for adding as a TMS image layer to JOSM or iD):

https://tile.opengeofiction.net/ttopo/{zoom}/{x}/{y}.png

Maximum zoom is 19.

It's "layer code letter" is "V" (this is a suffix to the URL, e.g. "...&layers=V").

CyclOGF

You can find the tiles at this path (e.g. for adding as a TMS image layer to JOSM or iD):

https://tiles06.opengeofiction.net/cyclogf/{zoom}/{x}/{y}.png

Maximum zoom is the same as for CyclOSM: zoom=19

It's "layer code letter" is "6" (this is a suffix to the URL, e.g. "...&layers=6").

Map updates

The CyclOGF and Tracestrack render databases receive incoming replication from the main apidb (opengeofiction.net) which currently "ticks" about every minute. However, low zooms (less than 10) are refreshed twice daily.

How can I add my own work to this above list of topo layers?

Contact Luciano via direct message in OGF, or send me an email (luciano🐌geofictician·net) or a ping in OGFC (OGF's discord server). We can discuss. First and foremost, do not upload contour lines or data to the OGF map directly! That's not how it works.

In the past, I've often said, "Send me a draft contour line file in in .osm format" (i.e. something drawn using JOSM, saved, and sent to me). But I'm going to change this workflow. I've received too many poorly done or frustratingly incomplete contour files, and so here's how I want to do this, moving forward.

First, we will discuss the area where you want to implement contours, and then I will send you one or more degree-square contour files as a sort of template, which you can then "fill in" with your contour lines. Then I can be sure to get the contour files in a format that is actually useful for the conversion program, and that is divided up into the degree-square format that I want to use to standardize this process.

Is the raw OGF topo data accessible online?

Yes, it is, on our data server. This is where the raw "OSM contour" files reside, and where the pseudo-SRTM degree squares are stored. The data is organized according into folders:

data.opengeofiction.net/dem - a folder per zone, containing:
- Processed contours, in .osm.pbf and .gpkg.zip formats
- DEM in TIFF .tif format
- Various hillshades and shaded reliefs in TIFF format
- .hgt files, in NASA SRTM HGT format
- A backwards-compatible .zip archive with files compatible with Thilo's old process output
- For example, the "roantra" zone: [1]
data.opengeofiction.net/dem/osm-squares - a folder per zone, containing:
- The input contour files, as created by OpenGeofiction users
- These are in .osm.xz format, which is natively supported by JOSM
dem-build-log.txt - output from the automated daily process
active-zones.txt - list of active zones

The osm-squares are the files you should download if you want to see what a "contour file" looks like in the JOSM editor, and, if you are in charge of maintenance of a contour zone, these are the files you should edit, alter, add detail to, and send back to me for re-upload.

If, on the other hand, you're interested in building your own topo render for the OGF world, you can use the other categories to make your job much, much easier, using off-the-shelf software stacks built to consume NASA-style SRTM data.