Striving to be the person my dog thinks I am.

One of the applications for photographic images acquired during Low Altitude Area Survey (LAAS) is Google Earth (GE).  The tool used to configure the GE platform is known as Keyhole Markup Language (KML), and  has numerous elements used to achieve different types of functionality.  The specific element used to overlay an image onto the GE terrain is <GroundOverlay> and it’s worthwhile reviewing how it is used, as it will dictate the type of data that needs to be collected by the vehicle when it’s in the air.

Note: No attempt will be made here to explain the intracies of KML — that is covered in lots of detail elsewhere, and the rest of this document assumes reasonable familiarity with it.The <LatLonBox> tag of the <GroundOverlay> element is used to drape an image onto the GE terrain.  It contains five of its own tags; <north>, <south>, <east>, <west> and <rotation>.  <north> and <south> are expressed in decimal degrees of latitude, and <east> and <west> are expressed in degrees longitude.  By convention, values north of the equator are considered positive, and south of the equator are considered negative.  Similarly, values west of the Prime Meridian are negative, and east are considered positive.  Finally, the <rotation> tag is expressed in decimal degrees from north in a counterclockwise direction.  An example a fully populated <LatLonBox> tag is as follows;

<LatLonBox>
	<north>48.25475939255556</north>
 	<south>48.25207367852141</south>
 	<east>-90.86591508839973</east>
	<west>-90.8714285289695</west>
	<rotation>39.37878630116985</rotation>
</LatLonBox>

The diagram at the top of this article illustrates the relationship of this tag information to the vehicle in the air.  Sufficient information has to be gathered during flight which can directly be employed in the population of the tag, or used as a basis for the calculation of values which, when combined with other known values and assumptions, can be used to populate the tag.

It’s assumed that the latitude and longitude of the vehicle can be precisely obtained using an inexpensive WAAS-enable Global Positioning System (GPS) device.  Previous posts describe the importance of keeping the camera oriented so that it is pointing precisely plumb (straight down).  Assuming this can be achieved, the vertical line from the camera to the terrain forms the adjacent side of a right triangle.  There are small, solid-state devices  capable of reporting altitude to a precision of one or two percent, so it’s reasonable to assume altitude above ground level (AGL) can also be obtained.  This is the length of the adjacent side of the right triangle.  Taking rotation out of the equation temporarily, the only other information required is the angle of view of the camera being used.  By taking half the angle of view, and combining it with latitude & longitude and AGL, this is the information necessary to calculate values for <north>, <south>, <east>, <west> using standard, garden-variety trigonometry (see SOHCAHTOA).

This is not fundamentally changed by the the presence (or absence) of information to populate the <rotation> tag.  In fact, if it could be assumed the vehicle was always headed precisely north, the <rotation> tag could be populated with zero, and otherwise ignored entirely.  However, it is assumed it will be easier to collect the heading — probably from the same GPS device that captured latitude & longitude — than to attempt to keep the aircraft pointed due north during image acquisition.  The only other thing that’s worth mentioning is the values collected in the air needed to be precisely correlated with respect to time.  It only makes sense that the lat/long, altitude and heading need to be recorded contemporaneously if they are going to be used as a basis for calculations.

Summary   The values required to prepare a syntactically correct GE <LatLonBox> tag are 1) altitude above ground level, 2) heading of the aircraft, and 3) latitude, and longitude of the vehicle, all at the time the picture is taken.