Trigonometric and Navigation Aid Points
Points with accurately known geographic coordinates are useful for estimating errors if they are visible in imagery acquired from Google Earth or Bing 3D. They are also helpful for the reduction of these errors as described elsewhere in this documentation.
Choose the desired grid in Setup if you have not already done so. Enter known coordinates in the dialogue box and click OK. The height value is the nominal Google Earth viewing height. Raising it shows a larger area, reducing it, a smaller one. The displayed area may be changed by zooming in or out with the altitude slider when the GE image is displayed. Reasonably accurate zooming is also possible with the numeric keypad PgUp/PgDn or the -/+ keys. Zooming with the mouse wheel in GE may reduce accuracy when the altitude is small, since Google's automatic image tilt can not be compensated precisely.
Trigonometric points or GeoPortal points for accurate calibration:
The appropriate grid must be chosen in AirPhotoSE's EPSG Option or as UTM WGS84. As an example, for France, there are three major grid systems, the oldest dating back to the 19th century and completed between the two world wars is the Nouvelle Triangulation de la France or NTF. Most paper maps still use it. In "Find in Grid" dialogue, the coordinates should be entered by copying them directly from the appropriate page of French Géoportail web site and pasting the numbers into the fields as shown. Other countries may have more than one grid or multiple zones too. Although the grid can be chosen in the EPSG Selection window, you must always choose the zone or the wrong area may be displayed.
Note that NTF divides France into four zones from north to south, so the zone number must also be specified or the values are ambiguous. A "flying height" which will show most of the area around the trig point nearest to an archaeological site should be chosen between 1000 and 5000 meters so that the image will include the trig point and the archaeological area. AirPhotoSE then displays the area centred precisely on the coordinates coming from Géoportail and calibrates the image automatically with the chosen grid. The coordinate value entered in the dialogue box will be the main calibration point thus making it unnecessary to visually identify the trig point in the GE image. Four further calibration points are added automatically to the corners of the image.
Some countries like France use commas instead of decimal points as separators. In the example above, if you are in a country which uses decimal points but you are working with trig point data that uses commas, copy the value to the left of the comma and paste it in the dialogue box, then copy or type in the values to the right of the decimal point.
Trigonometric points are usually quite difficult to see, since they are small and sometimes covered by vegetation or hidden in the shadows of buildings or trees.
NAVAID (navigation aid) points for calibration:
A NAVAID defines a point in space used for navigational and air traffic control purposes. NAVAIDS include VOR, DME, TACANs and others. Those with high power antennae are much easier to see than trigonometric points or the low power navigation aids.
Their latitudes and longitudes are given in lists prepared during the first decade of the 21st century, but these are no longer maintained. Several useful ones are available:
This is a comma separated variable list of over 11,000 points from all over the world with specified latitudes and longitudes and details which can be loaded into an Excel or Access compatible programme for searching and sorting. It can also be searched with a simple editor like Notepad somewhat less conveniently..
This list all the sites in somewhat more convenient form.
The Web site:
displays many of them along with airports on a Google Maps/Satellite background as symbols.
Usually the VOR-DME, TACAN and VORTAC antennae are very visible in Google Earth, but the lat/lon coordinates taken from the tables and used for a coordinate search in AirPhotoSE may give results which may be in error, depending how Google has stitched the images from an area.
The latitudes and longitudes in the tables above are given with as much as 18 decimal places. These are probably computational artefacts produced by the software used to construct those tables. The true precision is far less.
For better accuracy, a GeoPortal should be used if available to get precise coordinates. The coordinates from the tables above make a rough search in a GeoPortal easier. It will usually be to within 5 meters of the position of the antenna for points near major airports in developed countries. If the GeoPortal shows the coordinates in one of the many supported AirPhotoSE grid systems, then this should be used instead for calibrating the image by entering it into the AirPhotoSE coordinate search dialogue with coordinates chosen in Options to decimal lat/lon to get the best geo-referencing. Even then, errors in the stitching of Google Earth imagery may cause displacements of several meters.
Seen from the ground, the high power VORTAC/TACAN antennae are often very large structures surrounded by a circle of secondary antennae:
from Wikipedia, tilt corrected, reference above.
The coordinates are those of the centre of the circle of smaller antennae which can easily be estimated visually. Note that the DME antenna, the vertical white cylinder, is displaced from the centre of the ring in this instance and should not be used as the centre..
In the example shown here, coordinates in the Gauss-Krueger German grid were read from the mouse position at the centre of the antenna ring in the GeoPortal of Baden-Württemberg. Using EPSG code 31467, values are shown with 10 cm. resolution.
A search with these coordinates in AirPhotoSE's Google Earth implementation places the automatic central calibration point at about 3 meters south from the centre of the antenna circle, even though Google used the same image as that shown in the GeoPortal..
This error is typical of Google Earth, where images are stitched manually. Breaks in straight roads, airport runways and other linear features are often visible when the data comes from satellite imagery. Other sources of locational error in the navigation aids are in the lat/lon coordinates in the Web lists themselves, where original survey precision and choice of datum may vary. If images from different dates are available, as shown in the Google Earth time line, the errors in stitching or initial geo-referencing, especially of earlier images, may be much larger than those referred to above.
The images below using values from the North-Rhine Westfalia TIM GeoPortal (the blue pin) and from the NAVAID tables (the red pin) of the Cologne-Bonn Airport VOR-DME show a typical error when using a lat/lon pair from the NAVAID tables:
Searching with the GeoPortal lat/lon values:
Searching with the table lat/lon values:
The table lat/lon errors are combined with the Google Earth stitching errors when using GE.