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The ability to interpret aerial imagery and to define a point of interest in a coordinate system or map is critical to daily activities regarding elections, government, navigation, public safety, radio communication, terrain profiling, and weather, to name a few. While two dimensional mapping products are prone to error, they are standard for use because they remain familiar and inexpensive to create and publish.
In communications, aerial placement and frequency are often selected in consideration of the terrain and topography through which the signal will pass. So, geolocation and terrain profiling greatly depend on the use of accurate positions. Some frequencies work better in direct, line of sight applications like amateur television (ATV). Terrain may block radio interference or prevent a radio communication from taking place. Geolocation and terrain profiling offer a way to consider what is possible to do with the environment that is available. They help us to analyze, exploit, and predict alternate paths of radio communication apart from a direct route.
Accuracy and precision improve over time with maps, software, and source imagery. Release of a new version often results in a "datum shift" that renders older coordinates unusable for their intended purpose in modern times. Datum shift can also occur when world bodies meet and set policy (and standards) for how to communicate geographical information for the benefit of all people. A new coordinate system may be introduced for worldwide use, or existing coordinate systems become modernized with new datum and modeling. Two examples of datum include North American Datum of 1927 (NAD27) and North American Datum of 1983 (NAD83), which are currently in use amongst the Federal Communications Commission (FCC), National Oceanic and Atmospheric Administration (NOAA), National Map, and US Geological Survey (USGS).
Mathematics can be applied to sets of old coordinates to update them in response to datum shift, but this is rather like performing surgery while reading a magazine at the same time. The work cannot be relied upon to achieve the desired outcome when proper attention is lacking.
A simpler way to take old location data and make it current and relevant for its intended purpose is to go to the latest authoritative map and look it up all over again when the need arises.
Persons often keep digital and paper records with outdated coordinates on the presumption that they will serve as a useful reference at a future time. They are not wrong because contractual, historical and legal obligations are fulfilled when persons archive records. Therefore, outdated coordinates survive for years, and we must be prepared to recognize them whenever we encounter them. Our knowledge of old and outdated coordinates and coordinate systems continues after the old is retired and replaced with new ones, so there is a way to convert between such systems if one must go back. Just as important is the need to resurvey and update property boundaries and national borders. Such survey work demands records of older coordinates and records of geomagnetic reference models that were in effect at the time of the original survey. We may still choose whether to apply mathematics or look them up again to get it right. Important concerns with such archives are to comply with law, to introduce accurate and precise data, to prevent future confusion, and to save time.
Three dimensions are often captured in overhead imagery. Some of the captured spatial relationships among features gets left out when the source image is distributed widely in a two dimension format, as happens on the Internet.
The position of the sun is recorded in aerial imagery and overhead photographs for mapping. Because there is only one point for our sun, all its rays strike the Earth from one predominant angle, creating distinctive light and shadow patterns that our eyes discern in photographs.
We expect all shadows in source imagery to flow parallel to the direction of the light source, helping our eyes to hone in on our object of interest. But, multiple source images are also possible. A mosaic may introduce variation on light and shadow due to differing dates, times, and weather represented in the work.
High tension wires, lamps, telephone poles, and trees look very much like their shadow counterparts, and this invites confusion and error in fixing a location and interpreting features. Low lying, man made objects (like buildings and vehicles) are easy to discern and help to keep focus in the source image. It is very important to remain aware of the sun's position and shadow flow in the image source.
Shadow is often the first lead that we have to locate an object of interest.
Broad, thick, and wide shadows tell more about the height and size of structures and features, as well as positions of the sun and camera at the time of the photograph. Photogrammetry is the study of the mathematics and other factors (like camera altitude, focus, and lens) that relate to these perspectives.
Shadows must be traced all the way back to their source point on the Earth in order to get an accurate position fix on a map. Multiple, parallel shadows can cluster, point, or trace back to a center mass point on the Earth that is appropriate for reading coordinates. A wide shadow that abruptly begins at a structure can be halved to get near to the correct coordinate.
To read a coordinate from the top of a high structure in a source image invites error. Consider the top of an antenna mast or the roof of a tall building, which is obviously high up in the air. Whatever point on the earth the angle of the two dimension photograph places in proximity to the top of the structure is the Earth coordinate that will be read on many Internet imagery and mapping products. To avoid such error, follow high structures down to their position at ground level and read coordinates there, using shadow clues to home in further on the center mass point. Lower features and structures that remain near ground level, like cars and houses, do not invite such error.
Small cement pads on the ground are common for anchoring guy wires and for propping up structural legs toward the exterior of wide structures, but there may also be one cement pad at the center mass point, as in the case of a tall, thin radio tower or a low, wide satellite dish. Such features make it easy to select the proper point for reading coordinates, especially when they have a rounded or rectangular shape. Preplanned construction often results in placement of structures on such cement pads near their center.
A straight edge (like a piece of paper, pencil, ruler, or taught string) is handy for reading the best coordinate from source images and maps. For example, shadows can be traced to where they all begin, cluster, or point in a source image. Two straight edges can span a wide rooftop from opposite corners to mark a center mass point on low structures. Or two straight edges can mark the center point at ground level between opposite legs of a communications or electrical transmission tower. Such techniques prevent confusion and improve the resulting coordinate being read. Overhead photographs are taken from different angles and shadow clues may not be enough, so a straight edge offers another way to mark the best point.
A sharp, crisp pointer arrow or cross hair on a monitor display is important to getting an accurate read of coordinates displayed on devices with Internet access. Blurry, bulky, and fuzzy pointers will inject error in selecting a point to be read. Generally, such pointers focus on the upper left of their mass. Internet sources of maps and aerial imagery will offer a coordinate system displayed with the mapping product, or they offer an application whereby a cursor and mouse click is used to get coordinates for a specified point.
Some websites and applications offer single or multiple icons that the user adds to the map display to serve as markers to focus on points of interest. Such icons can be selected, read, and moved around for even more precision at points of interest. Still other websites offer more sophistication in the form of a running display of coordinates that the cursor hovers over. Zoom and the ability to switch between map layers and overhead imagery are other common features. Some websites offer more resolution and zoom options than others do for the same location. A newer feature is the rendering of 45 degree angle overhead images that offer an alternate perspective on detail and features. It can become distracting and get in the way of standard overhead perspectives if left enabled.
Environment Canada's Weatheradio Canada network and National Oceanic and Atmospheric Administration's NOAA Weather Radio (NWR) broadcast weather alerts, conditions, and forecasts to their populations continuously. Geographical separation between American and Canadian weather radio transmitters enables sharing of seven discrete VHF radio channels, as well as streamlined manufacture and marketing of weather radio products in both countries.
