TENEVIA FLOWSNAP^®
STREAM GAUGING MEASURING SYSTEM THROUGH IMAGE ANALYSIS

The principle of gauging through image analysis proposed by FlowSnap is based on identifying movements on the water’s surface like, for example, small floating elements (branches, leaves, bubbles, etc.) or structural movements of the water.

To obtain flow velocity measurements on the user’s selected section, the system is implemented in two steps:

• Calibration: This step can be carried out just once, either before or after acquiring a measurement video.
• Measurement: Acquisition and processing of video sequences taken from a nearby location to the one chosen for the calibration.

In real terms, these two steps consist of actions that are carried out on the site and processing done using the software.

This step consists of matching points whose topographic coordinates have been surveyed in the field with their position in a calibration image. These points can be fixed markers and temporary markers, only materialised during the calibration process. The recommended equipment for this step is the Leica S910 laser distance meter available in the FlowSnap equipment section. This calibration allows to convert the measured velocities in pixels per second in a metric unit (or Anglo-Saxon) whatever the water level is.

Once the image is in a 3D reference frame (X, Y, Z coordinates), it is necessary to set up a transverse profile to obtain flow information. The interface of the software allows a simple fusion of topographic and bathymetric data.

In addition, the software offers the possibility to visualise in 2D the horizontal and vertical interactions with the different calibration points.

The action of taking images consists of taking one or several videos or sequences of images (Time Lapse) of the river that correspond to the moment when the measurements are required.

Images must be taken by a device installed on a base so that the video or sequence of images is not corrupted by the device moving.

The image-taking therefore corresponds to the “field part” of a series of stream gauging in FlowSnap.

The measurement is done by processing the images in the FlowSnap 2 software. Once all the videos or image sequences have been taken on site, there are only a few steps to obtain flow information.

1. Upload the images or video (image extraction)
2. Choice of calibration
3. Specifying the height of the water body at the time of image capture.
4. Recalibration between the measurement images and the calibration image
5. Processing the velocities on the selected image pairs
6. Calculation of the flow rate

The processing of the velocities (5) can be carried out automatically by the software and manually by the user. This treatment makes it possible to obtain a very great number of velocity vectors on the section (several hundreds to several thousands), visually controllable on a “result image”. The design of the software also favours the archiving and management of the various measurement campaigns carried out on the same site, thus making it possible to constitute a measurement history for the development of a rating curve.

TRAINING OBJECTIVES

The aim of this training is to present the general principles that allow the transfer from the image (information in pixels) to a real value (metric information), using measurements taken on the site. The training also aims at giving user guidelines in order to take usable pictures and thereby ensure greater precision. The training is for professionals or those who use river measurements, without any particular prerequisites.

PROGRAM

• Introduction to the technique of image analysis (indoors) [theory]
• On-site session and using the recommended equipment [remote assistance on a first site]
• Presentation of TENEVIA’s FlowSnap® software [using the software]

Processor: Intel® or AMD processor with 64-bit support, 2 cores; 2 GHz processor or higher
Operating system : Microsoft Windows 10 (64 bits)
RAM : At least 4 GB RAM (8 GB recommended)
Disk Space : At least 2 GB of disk space for installation. Plan for additional space for calibrations and images taken by the user.
Connection : Internet connection required for software activation

Over and above the site’s hydrological characteristics, the choice of an appropriate section must validate the following points:

• safe accessibility to the site during floods
• the presence of long-term fixed points that are
• visible on the images (required for calibration)
• the existence of a reference staff gauge or fixed
• point out of the water whatever the flow conditions to determine the depth of water
• visually perceptible flow
• positioning of the surface velocity measurement zone at the centre of the image
• the visibility of both banks on the image

It is also important to anticipate the growth of vegetation that may present an obstacle to future image taking.

The average calibration time is about an hour. This includes the acquisition of the terrain data on site and its processing on the computer. The calibration only needs to be done once on each site, before or after the first images are taken.

The processing time of a sequence of images can vary considerably according to the user’s experience and the size of the site. The average time observed for optimum processing of a sequence of images is about 10 minutes. The image acquisition time on the site can be as little as two minutes.

The equipment proposed by TENEVIA in the PHOTO PACK is splash-proof. It offers the advantage of being splash-resistant and takes pictures of a very high quality (4K video). Images with good resolution allow FlowSnap to be used more effectively.

However, it is certainly possible to use another camera or image-taking device (smart phone, video camera).

• Continuous shooting mode
  For use in “continuous shooting” mode, the device must record the Exif metadata in the image file. The accuracy of the images timestamps should be below 10 milliseconds. It is also recommended that the maximum period between two images should be 100ms.
• Video mode
  To use video mode, we recommend using an image-taking frequency of at least 10 images per second.

Tip: Using a tripod when taking images is highly recommended to prevent interference within a sequence of images.

Yes, TENEVIA FlowSnap® can import topometric surveys in the form of CSV files including XYZ coordinates of the measured points.

The advantage of the equipment recommended by TENEVIA in the TOPO PACK is that it associates a photo thumbnail with each topographic point measured. This makes it easier to match them with the points of the calibration image.

If other topographical survey equipment is used, the order of the points must be carefully noted to be sure that they can be identified with certitude in the calibration image.

The measurement precision depends on the image resolution used and the calibration precision. Strictly respecting the on-site protocol optimises the measurements taken and their use. An aid confirming the pertinence of the geographical matrix created by the user is incorporated in the FlowSnap software.

No, the calibration phase only needs to be carried out once per site. This step can be done before or after the image taking.

A calibration has to include around 10 topographical points spread over both sides of the river. Some of these points must be fixed so they are always identifiable in the image. The others can be temporary markers placed on the site on the day of the calibration survey.

Yes, FlowSnap can be installed on a laptop. This means, for example, that the points used for the calibration step can be directly validated on the site. Its ergonomy means it can be used in the field.

The FlowSnap licence is valid for a single computer. However, several licences can be ordered.