GPS Recording

From ActiveIDsWiki

Aim & workflow

The aim of this tutorial is to get a basic understanding of working with and creating GPS recordings. For this tutorial you need a gps tracker, preferably a photo camera and optionally other recording devices. You will first get a background on GPS recordings and will then learn how to synchronize your gear, create a recording and see what you can do with this recording.

Tutorial requirements:

• GPS tracker
• photo camera (optional)
• other multimedia recorders (optional)

Global Positioning System (GPS)

source: Wikipedia - GPS

Gear

GPS trackers

A GPS tracker is a GPS receiver that records it's position at a set interval, precisely calculated based on the signal from the GPS satellites. GPS receivers are available integrated in many devices. A device which simply records the position, but doesn't have a display user interface is commonly referred to as a GPS tracker.

The receiver is the most important feature of the GPS tracker as the quality of the recording is determined by the quality of the reception. The quality of the reception also depends on the physical circumstances. A receiver generally works best outdoors, in an open surrounding and a clear sky.

Within this tutorial the workflow for working with GPS trackers will be explained in general and specifically for two GPS trackers, the Sony GPS CS1 and the RoyakTek RGM-3800. Both trackers are very affordable (between 50 and 100 euros).

Sony GPS CS1

The Sony GPS CS1 tracker is very easy to use. It has a single button to switch on/off and check the battery status. A clear LED indicator shows the device (recording and battery) status. The tracker has a mini usb interface and does not require any software (plug-and-play).

• general: specifications
• manual: online manual (pdf)
• drivers: (not required)
• software: (not required)
• filetype: *.log files; NMEA0183 format

To start recording with the Sony GPS CS1 tracker press the on/off swich. When the device is switched on it will try to determine it's position, for which it needs at least signals from three different GPS satellites. The receiver in the CS1 is not very strong and thus you might need to keep the physical circumstances (surroundings, weather) in mind.

When the tracker is searching for satellites and has not enough signals (yet) the LED indicator will constantly blink twice quickly. When the tracker has enough signals it will start recording it's position (longitude, latitude and altitude) and will constantly blink once slowly. When you want to stop the recording you press and hold the on/off switch until it turns off. The recording is saved to the internal memory automatically.

To obtain the file containing the GPS recording, connect the GPS tracker to your computer and explore the usb drive. On the tracker you will find the GPS recordings as files with a *.log extension (in NMEA0183 format). Copy these files to your computer. You can erase the files from the tracker by deleting them in the explorer or by using the reset button (hidden under the cap) on the tracker itself.

RoyalTek RGM-3800

The RoyalTek RGM-3800 tracker is very easy to use. It has a single button to switch on/off but unfortunately has a weak LED indicator. The tracker has a strong receiver. The tracker has a mini usb interface and needs software to connect to your computer (no plug-and-play) and to transfer files.

• general: specifications, intro guide (pdf)
• manual: online manual (pdf)
• drivers: Win 98/ME/2000/XP/Vista 32bit, Win Vista 64bit
• software: datadownloader
• filetype: *.nmea or *.txt files; NMEA0183 format

To start recording with the RoyalTek RGM-3800 tracker press the on/off swich. When the device is switched on it will try to determine it's position, for which it needs at least signals from three different GPS satellites. The tracker has a strong receiver so once it has determined it's location you might be able to keep tracking while your inside a vehicle.

When the tracker is searching for satellites and has not enough signals (yet) the LED indicator will constantly blink. When the tracker has enough signals it will start recording it's position (longitude, latitude and altitude and optionally speed and direction) and will be lit constantly. When you want to stop the recording you press and hold the on/off switch until it turns off. The recording is saved to the internal memory automatically.

To obtain the file containing the GPS recording, connect the GPS tracker to your computer. When you have the driver installed run the datadownloader software. Use the software interface to copy the files to your computer. You can choose to export the GPS recordings either as *.txt (very readable) or *.nmea (in NMEA0183 format, needed for geocoding) files. You can erase the files from the tracker using the software interface or by using the reset button (hidden under the cap) on the tracker itself.

Multimedia recorders

The geo data created when making a GPS recording can be added to and combined with many multimedia data (eg. photos, videos, audio recordings). The easiest is adding geo data to images (geocoding images) but is also possible with other multimedia data. To create multimedia data you use multimedia recording devices such a a photo camera, a video camera, a voice recorder, sketchbook, notebook or any other you can think of.

Photo camera

A photo camera is a great tool to combine with a GPS tracker because you can easily use geocoding software to add the geo data (location) from a GPS recording to a photo.

As background information: For the geotagging of images the photos taken have to support Exchangeable Image File Format (EXIF). This is generally the case for all digital cameras. Alternatively a photo camera can support IPTC and EMP.

A new generation of photo cameras has a build-in GPS receiver automatically storing the location into each picture taken.

Other recorders

Next to your photo camera you can also use other multimedia recording devices to record video, audio, notes, sketches and more. However Geocoding multimedia data (adding or combining with geo data from the GPS recordings) is generally more difficult than geocoding photos. When you are new to geocoding stick to geotagging photos and possibly add other multimedia data to your collection later.

Synchronizing your gear

In order to geotag your photos your camera needs to be able to safe the time when the photo is taken into the photo's EXIF data. This is generally the case for every digital camera. The time a picture is taken must correspond to the time the coordinates are saved by the GPS Tracker as geotagging software uses the time from the picture and looks in the track file to see what your location was at that time. The GPS Tracker uses GPS time. Make sure you set the time of your camera before you start geotagging. It's difficult to set the time of your camera exactly to GPS time, but you can use thetimenow.com or type in your location + the word time into Google search and you will get the time in UTC format.

When your done with tracking and start geotagging you might find that your pictures aren't taken exactly at the point you've taken them. This can have two reasons. The Sony GPS CS1 has a deviation up to about 10m. This is something you can do nothing about except buying a more advanced GPS Tracker. The second reason for the positions being off is that the time set in your camera does not exactly match the GPS time. The simplest way to fix this problem is by determining how far you where off your actual location in minutes / seconds. And change your camera time settings accordingly. For instance if you notice that your picture is shown on the map at a point where you actually were 1:30 minute earlier, your camera is 1:30 behind on GPS time, so you should set your camera time 1:30 minute forward.

Files

The geo data files created from the GPS recordings can for instance be visualized on a map (map tracks),to geocode images or to geocode other multimedia.

The GPS trackers produce files with different extensions (eg. *.log, *.txt, *.nmea) but are generally in the same NMEA0183 format. If you are uncertain a about the format you can check the specifactions of your tracker or simply open the file in a text editor.

Below is an example of the first couple of lines of a geo data file in NMEA0183 format recorded with a Sony GPS CS1 tracker:

The meaning of the highlighted line is:

$GP           Global Positioning
GGA           Global Positioning System Fix Data
014732        Fix taken at 01:47:32 UTC
1148.5378,N   Latitude 11 deg 48.5378' N
09947.7848,E  Longitude 99 deg 47.7848' E
1             Fix quality: 1 = GPS fix (SPS)
04            Number of satellites being tracked
15.1          Horizontal dilution of position
12.3,M        Altitude, Meters, above mean sea level
-26.9,M       Height of geoid (mean sea level) above WGS84 ellipsoid
(empty field) time in seconds since last DGPS update
*65           the checksum data, always begins with *

key source: gpsinformation.org

For more information see:

• nmea sentences
• position sentences

What's next?

With your GPS recording you can now do the following:

• visualize tracks on map
• geocode images
• geocode multimedia