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Json Web Service for XTide

xtwsd stands for "XTide Web Service Daemon", and provides a json based RESTful API to David Flater's famous XTide tide prediction software.


  • Retrieve tide and current predictions in json and svg format
  • Search for stations closest to a specific lat/long
  • Add new reference and subordinate stations via a Json POST


xtwsd is written in C++ versoin 11 and is compiled with CMake and has the following dependencies:

Project Description Source code
XTide The primary workhorse of this project, XTide's libxtide library provides the actual prediction calculations and data.
libtcd libtcd is a library that gives access to XTide's low level data (harmonics database)
nlohmann::json Excellent json library for C++
served RESTful framework for C++
boost C++ library used by served
openssl SSL library needed by nos2xt utility

Most of the above projects will be automatically downloaded and built as part of the checkout and build process. The one exception being the "Boost" library (which is a dependency of both served as well as the nos2xt utility described later). Boost version 1.70.0 or greater is required.


If you don't already have the Boost library installed in your development environment, download the latest code from and build the "system" library using the following commands:

cd /my/projects
tar zxvf boost_1_70_0.tar.gz
cd boost_1_70_0
./ -with-libraries=system

By convention, Boost development requires the environment variables BOOST_ROOT and BOOST_LIBRARYDIR to be set. Set those to values appropriate for your system. Here is an example, though your directories may be different:

export BOOST_LIBRARYDIR=/usr/local/boost_1_70_0/libs
export BOOST_ROOT=/usr/local/boost_1_70_0

Once Boost is built, you can download and build xtwsd with the following commands:

cd /my/projects
git clone --recursive
mkdir xtwsd/build
cd xtwsd/build

On most Linux environments, you configure the build with the following:

cmake -Wno-dev ..

On Mac OS X, issue the following instead:

cmake -Wno-dev -DOPENSSL_ROOT_DIR=/usr/local/opt/openssl ..

Once CMake has configured your build environment, you can build the system with a simple make command:



Be sure you have also downloaded a harmonics data file from the FlaterCo web site. Per the XTide specifications, you must then specify the path where your harmonics file is stored in either the environment variable HFILE_PATH, or the file /etc/xtide.conf. Finally, run xtwsd with the command: xtwsd <port>


xtwsd 8080

nos2xt utility

The United States National Oceanic and Atmospheric Administration (NOAA) provides public harmonics data via a web service interface for stations it maintains in the USA and parts of the Carribean. The stations from the USA are part of the harmonics database provided on the FlaterCo web site, but other stations in the Carribean are not. The utility nos2xt, (source code in src/util) can make calls to the public web service maintained by NOAA and make web service calls to your local xtwsd server to add those additional stations. That code also serves as a good example on how to use the xtwsd services for adding new data to the server using data you may find in other locations.

As an example, the following command can be run from the "build" directory to add missing Carribean tide stations:

./nos2xt -f ../src/util/caribbean.txt -p 8080

Type nos2xt with no parameters to see the options available to you.

KNOWN BUG: There seems to be an issue with doing "updates" to existing tide prediction data records. The crash occurs somewhere inside the libtcd library of XTide. The data seems to be saved, the the xtwsd server crashes. Restarting it seems to work. Adding NEW tide prediction records seems to work fine. For this reason, the default behavior of nos2xt is to skip tide stations that are already in the harmonics database. You can override this behavior with the "-u" option.

Station Index vs. Station Id

The XTide project (and in turn, the libxtide libraries) references stations using a number called an "index." This index is an index into an internal cache of stations. The biggest problem with using this index to uniquely identify a station is that is can change when new station data is added to the database. The tcd library keeps stations alphabetized and that sorting affects the index number. For this reason, whenever possible, xtwsd uses a synthesized id that is in the format <source.context>:<source.stationId>. Each tide and current station record has a "source context" that identifies where the prediction data came from. For example "NOS" is the United States' National Oceanic Service, which is a department within the National Oceanic and Atmosopheric Administration (NOAA). The NOS uses its own system to identify its stations. For example, the station id 8723178 is the station at Government Cut, Miami Florida. The "Station id" used by xtwsd thus would be NOS:8723178. Using this as an identifier ensures the station is uniquely identified between database updates.

In the resource paths below, where you see {stationId}, you can in fact use either the station Id or the station index. Just be aware that if you are using xtwsd in an environment where the station data may be added to or updated, the station index for any given station may change following such updates.

Resource paths

The following resource paths are available from xtwsd:

GET /locations</[tide|current]><?referenceOnly=[0|1]>

Retrieves a list of every tide or current station in the database. Requesting "/locations" by itself returns every tide and current station in the database. By specifying the referenceOnly parameter, you can limit the returned list to reference stations only. Because this list can be very long, you may be more interestd in the /nearest path described below.


GET /nearest</[tide|current]>?lat=n&lng=n<&count=n><&referenceOnly=[0|1]>

Retrieves a list of the tide or current stations that are closted to the specified latitude and longitude parameters. The parameters lat and lng should be specified in decimal degrees format (e.g. 28.1234). The five closest stations will be returned unless the count parameter is used to specify a different number. By specifying the referenceOnly parameter, you can limit the returned list to reference stations only.


GET /location/{stationId}<?start=YYYY-MM-DD HH:MM ZZZ><&days=n><&local=[1|0]><&detailed=[1|0]>

Retrieves the tide or current predictions for the specified station. If specified, start is the date the predictions will start. If not specified, today's date is used. If specified, days is the number of days beyond start to predict (default if not specified is 1). local determines if the times returned should be in the same time zone the station is (local=1) or GMT (local=0 or not specified). If detailed=1, predictions will include events such as sunrise, and moonrise, provided they occur within the specified time range. detailed=0 (or not specified) returns only high and low tide events.

Example 12:00 am EDT&local=1&detailed=1

GET /graph/{stationId}<?start=YYYY-MM-DD HH:MM ZZZ><&width=n><&height=n>

Returns an SVG graph of the tide or current predictions for the specified station, starting at the specified start date. The optional width and height can be used to specify the size (in pixels) of the returned SVG image.

Example 12:00 am EDT&width=1200&height=600

GET /harmonics/{stationId}

Retrieves the harmonic constituents for the prediction data for the specified tide or current station. If the station is a subordinate station, the harmonic offsets will be returned instead.


GET /harmonics/schema

Returns a Json Schema that specifies the required and optional data required to add or update new prediction data. This schema also covers the data returned by GET /harmonics{stationId}.


POST /harmonics

Allows new prediction data to be added to the database. The Json object sent with the POST request should match the Json Schema returned by GET /harmonics/schema. If you are updating an existing record, be sure the index property is populated. For adding new records, index should be omitted, or set to "-1". The response to the POST will be a Json object with the following structure:

    "statusCode": nn // The HTTP status code (200 = OK, 400 = bad request, 500 = internal error, etc.)
    "index": nnnn // The index of the record that was updated or added
    "message": "Details of any failures"


$ curl -vX POST -d @stationdata.json \
--header "Content-Type: application/json"

contents of stationdata.json:

    "comments": "",
    "country": "Bahamas",
    "harmonics": {
        "confidence": 10,
        "constituents": [
                "name": "M2",
                "amp": 1.329,
                "epoch": 10.6 
            ... see tests/ directory for complete data ...
        "datum": "Mean Lower Low Water",
        "datumOffset": 1.47,
        "zoneOffset": 0
    "levelUnits": "feet",
    "name": "Settlement Point Bahamas",
    "notes": "",
    "position": {
        "lat": 26.710,
        "long": -78.99666667
    "referenceStation": true,
    "source": {
        "context": "NOS",
        "name": "",
        "stationId": "9710441"
    "timezone": ":America/New_York",
    "type": "tide"

GET /tcd

Retrieves basic information about the harmonics data library currently in use.



Json Tide and Current Prediction Web Service for XTide







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