The following notes give the key steps in the data processing undertaken to convert from a GPS reading to a Chart Section and Chainage value.
The GPS device is activated from the HighLight Main Panel Display. HighLight displays a red triangle, alternating in the bottom corners of panel while acquiring satellite positioning, alternating green once a GPS location is acquired.
This GPS Link should be left open and running continuously in the background whilst the GPS is active and collects positional readings from the unit. This source data is given in decimal values for Longitude and Latitude only. The information held by HighLight at this point is:
Longitude and Latitude (Decimal values)
GPS Time
Number Satellites locked on to
Direction of Travel
Speed of Travel
At this time the location is not converted to an OS Grid Reference. Altitude is discarded as it is of little use to this application.
The Survey Display panel carries a timer trip that triggers at intervals of one second to monitor the current GPS location as held on the background GPS panel. [When the GPS is not available / suspended the timer trip falls back to a 15 second interval and sets the Time on the panel display from the device clock].
When a GPS reading is available, this is first converted to an OS Grid Reference. This step can only be achieved with access to the current database file (.sdf file) as the conversion requires details on the geographic transforms.
Database access is required for the OS Conversion when the device has moved through each 1km square of the underlying OS Grid - HighLight reads the transform data for the four corners of the current square.
At 50mph this data read happens every 40 seconds or so as the device will move through successive 1 km squares.
At walking speeds the movement between 1km squares is obviously much less frequent and HighLight is able to cache the current transform values to minimise database access. At 4km per hour walking pace the user will cross 1 km squares every 15 minutes or so.
If the OS Grid Reference conversion fails then HighLight determines that the device is outside the current Contract Area, (HighLight is configured to the target Contract Area, and not the entire UK) and the GPS based Survey processing will not progress.
HighStyle performs a quick check to see if the location falls within the gross area of the Survey (the rectangular area that covers all Chart Sections held in the Survey). This check includes a buffer area of 500m to allow for locations near the edge of Chart Sections. The GPS based Survey processing will not progress if the location is outside the gross Survey Area.
When the device is travelling in a vehicle (above 5 kph) HighStyle uses the primary direction to establish which Chart Section the device is now within (if any). If travelling on a N - S axis the range of the Section Northings are considered, but when moving on an E - W axis the Eastings are considered. The check runs through the Chart Sections in the order in which they are defined within the survey and the first match is picked out. A gross check is made against the opposite axis coordinates for consistency of the selection, this check has a built in 100m tolerance in either direction to allow for the 'width' (or curve) of the target network section.If travelling on a N - S axis the range of the Section Northings are considered, but when moving on an E - W axis the Eastings are considered. The check runs through the Chart Sections in the order in which they are defined within the survey and the first match is picked out. A gross check is made against the opposite axis coordinates for consistency of the selection, this check has a built in 100m tolerance in either direction to allow for the 'width' (or curve) of the target network section.
When travelling at walking speeds (under 5 km) the direction reported by the GPS is ignored in the location check (the direction is not reliable enough, the user may be just crossing the carriageway and not walking along it). In this instance a detailed check for location is made against the primary direction of the Chart Section itself. This uses either N - S or E - W axis directions as the primary selection criteria.In this instance a detailed check for location is made against the primary direction of the Chart Section itself. This uses either N - S or E - W axis directions as the primary selection criteria.
HighStyle tracks through the Chart Sections in the order listed in the Survey and always checks first in the last identified Section, then the one following etc. until a match is found. This approach minimises the time HighLight spends searching the full list of Chart Sections for a current position lock.
If HighLight detects that a Chart section has been skipped (for example a very short section length) then it will assume that the intermediate section has been traversed and will record as such.
This Chart Section check is valid only when the Survey Route is defined and followed. Gross deviations from the route will be detected, e.g. leaving the motorway mid-way through, and potentially picking up a leg of the survey in the opposite direction later in the Survey Route. However the source geographic data is not accurate enough to distinguish between adjacent Chart Sections such as parallel slips and carriageway sections, or where two routes diverge / converge. Identifying an accurate reference to Chart Section in these instances currently needs a semi-automatic approach, with the user making the final selection from a suggested prompt list. This aspect should be considered when creating network Survey Routes.
Chainage within the Chart Section is derived from a twin distance check to the node points at each end of the identified Section. The distance from the current point to both the Start Node and the End Node is calculated (calling on Pythagoras for the calculation), and the Chainage derived as the proportion of the Start Node distance divided by the sum of the two distances as applied to the recorded Chart Section Length from the main database definitions. This gives the most reliable 'quick calculation' of the mid-section Chainage, the value is shown on the screen display.
The above calculation considers just the Start and Finish Nodes for the current Chart Section. If a list of intermediate points for a Section are available (defining a sharp curve on a Slip for instance) then the calculation could work along this defining arc for a better match. However, such a detailed calculation is probably only worth the additional delay in calculation when the device is undertaking a walking or low speed survey. The detailed information to support this calculation is not currently available on most Contracts, but can be applied easily enough if the information is added to the main Contract Database.
The Marker Post distance is calculation from the Start Distance as recorded against the Chart Section, plus the Chainage value from above. The value is converted to a '43/567' format and hence shows a value to the nearest metre. There is, therefore, no direct correlation to the physical marker posts as found in the carriageway verge.
HighLight monitors the current time from either the GPS unit, or the device clock if no GPS is available. On screen panels this time is shown as '< 10:35' - indicating a (now passed) time of 10:35 today. Internally HighLight saves this as a date with times recorded to the nearest second. Note that when Survey records are saved to the main HighStone Database, these times are usually rounded and held to the nearest minute only, the seconds element of each time value is lost. This rounding of time values is not significant to the overall surveying application, but users should note this point when preparing reports etc.