The _OwnTracks Recorder_ is a lightweight program for storing and accessing location data published via MQTT (or HTTP) by the [OwnTracks](http://owntracks.org) apps. It is a compiled program which is easily to install and operate even on low-end hardware, and it doesn't require an external database.
There are two main components: the _Recorder_ obtains data via MQTT subscribes or HTTP POST, stores the data in plain files and serve it via its built-in REST API, and the _ocat_ command-line utility reads stored data in a variety of formats.
We developed the Recorder as a one-stop solution to storing location data published by our OwnTracks apps (iOS and Android) and retrieving this data. Our previous offerings (`m2s`, `o2s`/`Pista`) also work of course, but we believe the Recorder is best suited to most environments.
1. It subscribes to an MQTT broker and reads messages published from the OwnTracks apps, storing these in a particular fashion into what we call the _store_ which is basically a bunch of plain files on the file system. Alternatively the Recorder can listen on HTTP for OwnTracks-type JSON messages POSTed to its HTTP server.
2. It provides a Web server which serves static pages, a REST API you use to request data from the _store_, and a WebSocket server. The distribution comes with a few examples of how to access the data through its HTTP interface (REST API). In particular a _table_ of last locations has been made available as well as a _live map_ which updates via the Recorder's WebSocket interface when location publishes are received. In addition we provide maps with last points or tracks using the GeoJSON produced by the Recorder.
We provide a ready-to-run packages for a limited number of platforms on our [package repository](http://repo.owntracks.org/README.txt), and we provide a Docker image which bundles the Recorder and a Mosquitto broker [directly from the Docker hub](https://hub.docker.com/r/owntracks/recorderd/).
We create packages for releases for a few distributions. Please note that these packages depend on libmosquitto1 from the [Mosquitto project](http://mosquitto.org/downloads).
Binaries (`ocat`, `ot-recorder`) from these packages run setuid to user `owntracks` so that they work for all users of the system. Note that, say, certificate files you provide must therefore also be readable by the user `owntracks`.
The packages we provide have a systemd unit file in `/usr/share/doc/ot-recorder/ot-recorder.service` which you can use to have the Recorder started automatically:
1. Ensure you have a configuration file with the settings you require
3. Enable the service to run at startup: `systemctl enable ot-recorder`
4. Launch the service `systemctl start ot-recorder`
### Docker
We also have a Docker image to create containers which integrate a [Mosquitto broker](http://mosquitto.org) with the Recorder. The Docker image is [available from the Docker hub](https://hub.docker.com/r/owntracks/recorderd/) (e.g. `docker pull owntracks/recorderd`), and it's [usage is documented in the Booklet](http://owntracks.org/booklet/clients/recorder/).
You need a current version of libmosquitto (and you probably require the Mosquitto broker as well for OwnTracks). We strongly recommend installing Mosquitto either from [source](http://mosquitto.org/download/) or from a [binary package](http://mosquitto.org/download/), both of which are provided by the [Mosquitto project](http://mosquitto.org/). In particular, older or LTS OS versions profit from this.
On Debian, you can install the needed packages with:
1. Obtain and download the software, via [our Homebrew Tap](https://github.com/owntracks/homebrew-recorder) on Mac OS X, directly as a clone of the repository, or as a [tar ball](https://github.com/owntracks/recorder/releases) which you unpack.
2. Copy the included `config.mk.in` file to `config.mk` and edit that. You specify the features or tweaks you need. (The file is commented.) Pay particular attention to the installation directory and the value of the _store_ (`STORAGEDEFAULT`): that is where the Recorder will store its files. `DOCROOT` is the root of the directory from which the Recorder's HTTP server will serve files.
When _make_ finishes, you should have at least two executable programs called `ot-recorder` which is the Recorder proper, and `ocat`. If you want you can install these using `make install`, but this is not necessary: the programs will run from whichever directory you like if you add `--doc-root ./docroot` to the Recorder options.
Ensure the LMDB databases are initialized by running the following command which is safe to do, also after an upgrade. (This initialization is non-destructive -- it will not delete any data.)
The Recorder has, like _ocat_, a daunting number of options, most of which you will not require. Running either utility with the `-h` or `--help` switch will summarize their meanings. You can, for example launch with a specific storage directory, disable the HTTP server, change its port, etc.
Publish a location from your OwnTracks app and you should see the Recorder receive that on the console. If you haven't disabled Geo-lookups, you'll also see the address from which the publish originated.
When the Recorder has received a publish or two, visit it with your favorite Web browser by pointing your browser at `http://127.0.0.1:8083` or the address / port configured with the `--http-host` and `--http-port` options respectively.
Unless already provided by the package you installed, we recommend you create a shell script with which you hence-force launch the Recorder. Note that you can have it subscribe to multiple topics, and you can launch sundry instances of the Recorder (e.g. for distinct brokers) as long as you ensure:
`--port` is the port number of the MQTT broker and overrides `$OTR_PORT`; it defaults to 1883. Setting this to 0 disables MQTT even if it is compiled-in.
`--qos` specifies the MQTT QoS to use; it defaults to 2.
`--storagedir` is configured at build time and overrides `$OTR_STORAGEDIR`.
`--useretained` overrides the default of not consuming retained MQTT messages.
`--norec` disables writing of REC files, so no location history or other similar publishes are stored, and the Lua `otr_putrec()` function is not invoked even if it exists. What is stored are CARDS and PHOTOS, as well as the LAST location of a device. As such, the API's `/locations` endpoint becomes useless.
`--norevgeo` suppresses reverse geo lookups, but this means that historic data will not show addresses (e.g. with the API or with _ocat_). See below for information on Reverse Geo lookups.
`--logfacility` is the syslog facility to use (default is `LOCAL0`).
`--quiet` disables printing of messages to _stdout_.
`--initialize` creates the a structure within the storage directory and initializes the LMDB database. It is safe to use this even if such a database exists -- the database is not wiped. After initialization, Recorder exits.
`--geokey` sets the Google API key for reverse geo lookups. If you do more than 2500 (currently) reverse-geo requests per day, you'll need an API key for Google's geocoding service. Specify that here.
The Recorder attempts to read its startup configuration from a configuration file; the path to this is compiled into the Recorder (typically `/etc/defaults/ot-recorder`, and `ocat -v` will display the compiled-in default). The format of this file approximates that of a shell script with variables to be exported (the intention is so that it can be sourced by a shell script). Lines beginning with an octothorp (`#`) are ignored as are blank lines. Configuration settings proper are set as follows (note that some older versions of _libconfig_ require a trailing semicolon (`;`) at the end of a variable assignment):
The following configuration settings may be applied (a `Y` in column `$` means an environment variable of the same name overrides a setting in the config file):
Running the Recorder protected by an _nginx_ or _Apache_ server is possible and is the only recommended method if you want to server data behind _localhost_. The snippets below show how to do it, but you would also add authentication to them - or at least, to everything but the views. The snippet for HTTP mode shows an example of how to do this.
The Recorder has a built-in HTTP server with which it servers static files from either the compiled-in default `DOCROOT` directory or that specified at run-time with the `--doc-root` option. Furthermore, it serves JSON data from the API end-point at `/api/0/` and it has a built-in WebSocket server for the live map.
The API basically serves the same data as _ocat_ is able to produce - see [API.md](https://github.com/owntracks/recorder/blob/master/API.md). The server also accepts OwnTracks app data via HTTP POST to the `/pub` endpoint.
Retrieve the last position of a particular user. In addition to the values obtained in the [`location` publish](http://owntracks.org/booklet/tech/json/) from the OwnTracks device, there are a few which we return as convenience:
*`username` contains the name of the user obtained from the publish topic
*`device` contains the user's device name as obtained from the publish topic
*`topic` is the full topic to which the payload was published
*`ghash` is the geohash string which corresponds to `lat` and `lon`
*`isotst` is the ISO timestamp of the publish time (`tst`)
*`disptst` is the same but designed for displaying
*`cc` is the country code of the location point if available in the cache (see below)
*`addr` is the address of the location point if available in the cache
#### Display map with points starting at a particular date
By specifying a `format` we can produce GeoJSON, say. Normally, the API retrieves the last 6 hours of data but we can extend or limit this with the `from` and `to` parameters.
The Recorder's Web server also provides a tabular display which shows the last position of devices, their address, country, etc. Some of the columns are sortable, you can search for users/devices and click on the address to have a map opened at the device's last location.
The _ocat_ utility accesses _storage_ directly — it doesn’t use the Recorder’s REST interface. _ocat_ has a daunting number of options, some combinations of which make no sense at all.
prints data for the current month, starting now and going backwards; only 10 locations will be printed. Generally, the `--limit` option reads the storage back to front which makes no sense in some combinations.
Specifying `--fields lat,tid,lon` will request just those JSON elements from _storage_. (Note that doing so with output GPX or GEOJSON could render those formats useless if, say, `lat` is missing in the list of fields.)
The `--from` and `--to` options allow you to specify a UTC date and/or timestamp from which respectively until which data will be read. By default, the last 6 hours of data are produced. If `--from` is not specified, it therefore defaults to _now minus 6 hours_. If `--to` is not specified it defaults to _now_. Dates and times must be specified as strings, and the following formats are recognized:
The `--limit` option limits the output to the last specified number of records. This is a bit of an "expensive" operation because we search the `.rec` files backwards (i.e. from end to beginning). When using `--limit` the 6 hours mentioned earlier do not apply.
* The returned data structure is an array of JSON objects; had we omitted specifying a particular device or even a particular user we would have obtained the last position of all this user's devices or all users' devices respectively.
* If you are familiar with the [JSON data reported by the OwnTracks apps](http://owntracks.org/booklet/tech/json/) you'll notice that this JSON contains more information: this is provided on the fly by _ocat_ and the REST API, e.g. from the reverse-geo cache the Recorder maintains.
* Flat files. The filesystem is the database. Period. That's were everything is stored. It makes incremental backups, purging old data, manipulation via the Unix toolset easy. (Admittedly, for fast geo-lookups we employ LMDB as a cache, but the final word is in the filesystem.) We considered all manner of databases and decided to keep this as simple and lightweight as possible. You can however have the Recorder send data to a database of your choosing, in addition to the file system it uses, by utilizing our embedded Lua hook.
* We wanted to store received data in the format it's published in. As this format is JSON, we store this raw payload in the `.rec` files. If we add an attribute to the JSON published by our apps, you have it right there. There's one slight exception: the monthly logs (the `.rec` files) have a leading timestamp and a relative topic; see below. (In the particular case of the OwnTracks firmware for Greenwich devices which can publish in CSV mode, we convert the CSV into OwnTracks JSON for storage.)
* All times are UTC (a.k.a. Zulu or GMT). We got sick and tired of converting stuff back and forth. It is up to the consumer of the data to convert to localtime if need be.
* The Recorder does not provide authentication or authorization. Nothing at all. Zilch. Nada. Think about this before making it available on a publicly-accessible IP address. Or rather: don't think about it; just don't do it. You can of course place a HTTP proxy in front of the Recorder to control access to it. Or use views (see below).
*`ocat`, the _cat_ program for the Recorder uses the same back-end which is used by the API though it accesses it directly (i.e. without resorting to HTTP).
* The Recorder supports 3-level MQTT topics only, in the typical OwnTracks format: `"owntracks/<username>/<devicename>"`, optionally with a leading slash. (The first part of the topic need not be "owntracks".) Publishes via HTTP POST construct a ficticious topic internally using the provided user (`u`) and device (`d`) parameters.
As mentioned earlier, data is stored in files, and these files are relative to `STORAGEDIR` (compiled into the programs or specified as an option). In particular, the following directory structure can exist, whereby directories are created as needed by the Recorder:
*`cards/`, optional, may contains user cards. This card is then stored here and used with, e.g., `ocat --last` to show a user's name and optional avatar. User cards are typically stored in a subdirectory called _username_, and therein a JSON file _username_.json. When reading cards, the Recorder will first attempt to open _username_/_device_/_username_.json and then _username_/_username_.json.
*`config/`, optional, contains the JSON of a [device configuration](http://owntracks.org/booklet/features/remoteconfig/) (`.otrc`) which was requested remotely via a [dump command](http://owntracks.org/booklet/tech/json/#_typecmd). Note that this will contain sensitive data. You can use this `.otrc` file to restore the OwnTracks configuration on your device by copying to the device and opening it in OwnTracks.
*`ghash/`, unless disabled, reverse Geo data (using a Google service) is collected into an LMDB database located in this directory. This LMDB database also contains named databases which are used by your optional Lua hooks, as well as a `topic2tid` database which can be used for TID re-mapping.
*`last/` contains the last location published by devices. E.g. Jane's last publish from her iPhone would be in `last/jjolie/iphone/jjolie-iphone.json`. The JSON payload contained therein is enhanced with the fields `user`, `device`, `topic`, and `ghash`. If a device's `last/` directory contains a file called `extra.json` (i.e. matching the example, this would be `last/jjolie/iphone/extra.json`), the content of this file is merged into the existing JSON for this user and returned by the API. Note, that you cannot overwrite existing values. So, an `extra.json` containing `{ "tst" : 11 }` will do nothing because the `tst` element we obtain from location data overrules, but adding `{ "beverage" : "water" }` will do what you want. If Recorder is built with support for our Greenwich firmware, this directory might contain `batt.json`, `ext.json`, and/or `status.json` each of which hold an array of the last 100 reports for internal battery voltage, external voltage, and status respectively. These values are returned via the API in the LAST object. A file `http.json` which should contain either a single JSON object or an array of JSON objects is returned to clients in HTTPmode.
*`rec/` the Recorder data proper. One subdirectory per user, one subdirectory therein per device. Data files are named `YYYY-MM.rec` (e.g. `2015-08.rec` for the data accumulated during the month of August 2015.
*`waypoints/` contains a directory per user and device. Therein are individual files named by a timestamp with the JSON payload of published (i.e. shared) waypoints. The file names are timestamps because the `tst` of a waypoint is its key. If a user publishes all waypoints from a device (Publish Waypoints), the payload is stored in this directory as `username-device.otrw`. (Note, that this is the JSON [waypoints import format](http://owntracks.org/booklet/tech/json/#_typewaypoints).) You can use this `.otrw` file to restore the waypoints on your device by copying to the device and opening it in OwnTracks.
You should definitely **not** modify or touch these files: they remain under the control of the Recorder. You can of course, remove old `.rec` files if they consume too much space.
If not disabled with option `--norevgeo`, the Recorder will attempt to perform a reverse-geo lookup on the location coordinates it obtains and store them in an LMDB database. If a lookup is not possible, for example because you're over quota, the service isn't available, etc., Recorder keeps tracks of the coordinates which could *not* be resolved in a file named `missing`:
We recommend you keep reverse-geo lookups enabled, this data (country code `cc`, and the locations address `addr`) is used by the example Web apps provided by the Recorder to show where a particular device is. In addition, this cached data is used the the API (also _ocat_) when printing location data.
The precision with which reverse-geo lookups are performed is controlled with the `--precison` option to Recorder (and with the `--precision` option to _ocat_ when you query for data). The default precision is compiled into the code (from `config.mk`). The higher the number, the more frequently lookups are performed; conversely, the lower the number, the fewer lookups are performed. For example, a precision of 1 means that points within an area of approximately 5000 km^2 would resolve to a single address, whereas a precision of 7 means that points within an area of approximately 150 m^2 resolve to one address. The Recorder obtains a location publish, extracts the latitude and longitude, and then calculates the [geohash](https://en.wikipedia.org/wiki/Geohash) string and truncates it to _precision_. If the calculated geohash string can be found in our local LMDB cache, we consider the point cached; otherwise an actual reverse geo lookup (via HTTP) is performed and the result is cached in LMDB at the key of the geohash.
As an example, let's assume Jane's device is at position (lat, lon) `48.879840, 2.323522`, which resolves to a geohash string of length 7 `u09whf7`. We can [visualize this](http://www.movable-type.co.uk/scripts/geohash.html) and show what this looks like. (See also: [visualizing geohash](http://www.bigdatamodeling.org/2013/01/intuitive-geohash.html).)
Every location publish outside that very small blue square would mean another lookup. If, however, we lower the precision to, say, 5, a much larger area is covered
and a precision of 2 would mean that a very large part of France resolves to a single address:
The bottom line: if you run the Recorder with just a few devices and want to know quite exactly where you've been, use a high precision (7 is probably good). If you, on the other hand, run Recorder with many devices and are only interested in where a device was approximately, lower the precision; this also has the effect that fewer reverse-geo lookups will be performed in the Google infrastructure. (Also: respect their quotas!)
As hinted to above, the address data obtained through a reverse-geo lookup is stored in an embedded LMDB database, the content of which we can look at with
In order to monitor the Recorder, whenever an MQTT message is received, a `monitor` file located relative to STORAGEDEFAULT is maintained. It contains a single line of text: the epoch timestamp and the last received topic separated from each other by a space.
If Recorder is built with `WITH_PING` (default), a location publish to `owntracks/ping/ping` (i.e. username is `ping` and device is `ping`) can be used to round-trip-test the Recorder. For this particular username/device combination, Recorder will store LAST position, but it will not keep a `.REC` file for it. This can be used to verify, say, via your favorite monitoring system, that the Recorder is still operational.
After sending a _pingping_, you can query the REST interface to determine the difference in time. The `contrib/` directory has an example Python program (`ot-ping.py`) which you can adapt as needed for use by Icinga or Nagios.
A view is a sort of sandboxed look at data provided by the Recorder. Assume you host several devices, be they your own or those of some of your friends, and assume you want to allow somebody else to see where you are or have been during a specific time frame: with the Recorder's default Web server you cannot limit a visitor to see specific data only; once they reach the Recorder's Web interface, they have access to all your data. (We warned you about that earlier.) Using a HTTP proxy, you can provide an insight into certain portions of your data only.
You configure a view by creating a small JSON file of an arbitrary name which defines which user / device combination of data the view should display. Say you are recording data for `owntracks/jjolie/phone`, the _user_ would be `jjolie` and the _device_ is `phone`. You can also create a specific HTML page for this view or just use the default `vmap.html` we provide.
Suppose Jane wishes to have her acqaintances see where she is whilst on vacation. Jane knows she'll be en-route between 2015-06-29 and 2015-07-15. She creates a file called, say, `loire.json` in the `views/` directory of the Recorder's document root:
```json
{
"user" : "jjolie",
"device": "phone",
"page" : "vmap.html",
"from" : "2015-06-29",
"to" : "2015-07-15"
}
```
Jane's friends can now visit the URL `/view/loire` (note the missing `.json` extension) to be served a map showing Jane's progress along the Loire valley (if that is where she's actually travelling through). Jane can keep that view up even after she returns because the view will not serve data after the 15th of July, in other words, her location at any other time before or after the _from_ / _to_ dates is hidden.
It's recommended that you configure your reverse proxy to show views. You can find an example of this for nginx in the [reverse proxy](#reverse-proxy) section.
| auth | N | array of digest authentication tokens described below |
| label | N | text to use in popup of default `vmap.html` instead of user/device |
| zoom | N | zoom level for map used in `vmap.html`, defaults to 9 |
| * | N | any other element is copied into the data returned |
The _page_ is a single HTML file which must be located in the `views/` directory of the Recorder's document root. Trivial (primitive actually) text substitution is done for the following two tokens:
*`@@@LASTPOS@@@` is converted to a URI on which the Recorder will serve the last position data
*`@@@GEO@@@` is converted to a URI on which the Recorder will serve GeoJSON data from its storage.
The default _page_ we provide is called `vmap.html`; by default it refreshes the last position every 60 seconds, and clicking on _Load track_ loads the GeoJSON track for the time frame specified by `from` and `to`.
A little bit more complex view would look like this:
```json
{
"config": {
"port": 9001,
"pathname": "/tmp/somewhere"
},
"zoom": 7,
"label": "Jane's Loire vacation",
"to": "2015-07-15",
"from": "2015-06-29",
"device": "phone",
"user": "jjolie",
"page": "vmap.html"
}
```
All JSON elements are copied into the _lastpos_ data which is returned to the caller. Using the above view configuration, a user requesting `http://localhost:8083/view/loire?lastpos=1` would obtain
```json
{
"data": [
{
"_type": "location",
"cc": "FR",
"lon": -1.564214,
"lat": 47.217871,
"alt": 35,
"vel": 0,
"t": "L",
"cog": 0,
"tid": "K2",
"tst": 1436895718,
"ghash": "gbqus7u",
"addr": "Maison d'arrêt, 9 Rue Descartes, 44000 Nantes, France",
"locality": "Nantes",
"isorcv": "2015-07-14T19:41:58Z",
"isotst": "2015-07-14T17:41:58Z",
"disptst": "2015-07-14 17:41:58",
"page": "vmap.html",
"user": "jjolie",
"device": "phone",
"from": "2015-06-29",
"to": "2015-07-15",
"label": "Jane's Loire vacation",
"zoom": 7,
"pathname": "/tmp/somewhere",
"port": 9001
}
]
}
```
Note how `pathname` and `port` have been copied into the object. These values can be used by the _page_ served in the view.
If `view.json` contains an element called `auth`, it is assumed to be an array of strings, each of which are a 32-character [Digest authentication](https://en.wikipedia.org/wiki/Digest_access_authentication) SHA1 strings for the realm `owntracks-recorder`, for example:
In the above example, you copy the 32-character digest into your `view.json`, whereas in the following example, we create a template for you which you copy into your view.
If enabled at compile time (`WITH_HTTP`), the Recorder will accept OwnTracks-type JSON payloads via HTTP at the URL endpoint `/pub&u=username&d=device`. You specify the username with the `u` parameter and the device name with the `d` parameter. (Alternatively you can provide `X-Limit-U` and `X-Limit-D` as headers with the username and device name respectively.) If unspecified, the username defaults to `owntracks` and the device to `phone`. For example:
The content of the request is used by the Recorder as though it had arrived as an MQTT message; Lua hooks and WebSocket pushes are handled accordingly.
When a device posts a location request in HTTP mode, the endpoint may return a JSON array of OwnTracks objects of which `_type`s `cmd`, `location` and `card` may be supported by the device. This allows the device to see, say, friends. The Recorder has built-in support for this with the named "friends" lmdb database.
Assuming the following content of the friends database
when user `jane` and device `phone` POST a new location via HTTP, the Recorder will present John's data as the following payload to the device, assuming John has a card and last published this location:
In order to use the Recorder's maps and views, you have to obtain a [Google API "Browser key"](https://developers.google.com/maps/documentation/javascript/get-api-key). You then add this key to your _docroot_ directory so that the Recorder can find it. (You should already have a file called `apikey.js.sample` in that directory; copy or rename the file to `apikey.js` and ensure its content is valid JavaScript:
```bash
$ cat .../static/apikey.js
var apiKey = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
`ocat --load` and `ocat --dump` can be use to load and dump the lmdb database respectively. There is some support for loading/dumping named databases using `--load=xx` or `--dump=xx` to specify the name. Use the mdb utilities to actually perform backups of these. _load_ expects key/value strings in pairs, separated by exactly one space. If the value is the string `DELETE`, the key is deleted from the database, which allows us to, say, remove a whole bunch of geohash prefixes in one go (but be careful doing this):
This named lmdb database is keyed on topic name (`owntracks/jane/phone`). If the topic of an incoming message is found in the database, the `tid` member in the JSON payload is replaced by the string value of this key.
If the Recorder was built with encryption support (see below), this named database contains the secret decryption keys for users/device pairs. The LMDB key is the username followed by a dash followed by the device name, all lower case, with spaces translated to a single dash. For example, if user Jjolie with device iPhone needs a secret entered, the database key will be `jjolie-iphone`. This can be entered into the database as follows:
For http mode, the `friends` named LMDB database contains lists of "friends" on a per user-device key. The key's value must be a valid JSON array of strings, each in the form `"user:device"` or `"user/device"` which indicate which locations a particular user may see. For example, when a user called `jane` on device `phone` publishes in http mode and she should be permitted to see where `john` / `android` is, we add the following key/value to the friends named database:
If compiled with `WITH_ENCRYPT` support (this is the default in our packages), the Recorder will handle messages from OwnTracks [devices which support payload encryption](http://owntracks.org/booklet/features/encrypt/). Each user / device requires a secret key which is configured on the device and which must be configured on the Recorder host in order for the Recorder to be able to decrypt the payloads.
Upon successful decryption, the Recorder processes the original (device-transmitted) JSON and stores the result in plain (i.e. un-encrypted) form in the store.
It actually is possible to gateway location publishes arriving via HTTP into MQTT, though you should be careful not to create loops. You can accomplish this with one or more Lua hooks using MQTT from within Lua.
If a payload is received with an element called `_geoprec` it contains an overide for the Recorder's configured reverse-geo precision. So, for example, if Recorder is running with precision 7, say, and the received payload contains `"_geoprec" : 2` the 2 will be used for this particular publish. This is not used in the OwnTracks apps, but it can be used with payloads you generate otherwise. If `_geoprec` is negative, new reverse geo lookups will not be performed, but cached entries of `abs(_geoprec)` will be used.
