USING COORDINATES

The most helpful is valid coordinates in the form of latitude and longitude.  Oh if it was jus that simple!  Directories are best sources for lat and long coordinates but they can be off.  Those taken back in the 1920s and 1930s were generally rounded off.  The coordinates were normally given in degrees and minutes.  To begin to understand how coordinates can be off, you first have to appreciate the location from which the reading was taken.

For example, if you have a airfield that is 1 mile (5,280ft) square, if you take the reading at the upper left corner (see figure below) you get reading "A".  Move to the upper right corner, you are one mile from the first reading and now you get reading "B".  One mile equals approximately 1.5 minutes of arc.  Reading "C" at lower right corner increases the difference to approximately 1.75 minutes of arc.

Depending upon where the readings are taken, the airfield could be located in any of the adjoining gray squares too.  The numbers in the example diagram above vary based on latitude.  Thus, those used in the example have one minute equal approximately 0.94 mile (~4,784 feet) and illustrate the errors that can be introduced in coordinates.  Where readings are taken and from what references all contribute to coordinate errors.

Thus, when trying to get the exact location, these reading errors, plus round off can result in a ground error on the order of + / - ~1.5 miles.  Further errors can result from poor coordinates that the ground survey were taken from.  Normally, there are markers in an area that have been surveyed in are used for setting legal boundaries for property, roads, geographical items, etc.  The map/chart makers receive these coordinates from the airfield and use them to place the symbols on the chart.

Even today, you can check coordinates on AirNav or FAA's 5010 list, and find errors.  AirNav and FAA use data they receive.  So it is up to the supplier of the information, the airfield owner or manager to obtain accurate readings and supply them to the FAA, state aviation agencies, etal.  This is true even when the coordinates in lat and long are given to tenths of a second.  That is not to say that all readings are in error, but I would estimate that approximately 15% of the readings actually are not on or touching the airfield.

Thus, coordinates, while the best, are not infallible.  You might well need to look around if you do not find the airfield at the coordinates you are working with.  My search parameters is to look within 1.5 - 2 miles to see if I can locate the airfield.

Many directories and sources will also give the location of an airfield relative to a town or major known geographical location.  Again, there can be problem in locating the airfield with these.  Example: "located 1.5 miles east of city."  Was this measurement taken from the center of a city/town or the city limits?  Or some other reference.  Is it truly east?  Other references can help, e.g. north of highway and west or railroad tracks.

Elevation readings are useful to help resolve locations of airfields but you need to know that elevation readings can be off too.  Most elevation readings are estimates.  You can see a lot of variations in elevation for a given airfield.  What seems to happen is that a reading is used for a period of time till a new reading is taken and it is used for a number of years and so on.

The use of Global Position System (GPS) is improving coordinates and elevation numbers, assuming the person taking the readings uses a spot on the airfield to take the readings.

Most airfield definitions/descriptions often have a number of miles and heading from the associated town.  Many of these can be off.  Depends upon where the measurements are taken from.  Are they direct or driven miles?  The USGS generally used what is called the "center business district".  Directions are expressed in miles and compass headings such as north, south, east, west, and then in 45 degree increments of NE, SE, NW, SW.  Further subdivided into 22.5 degree increments to produced NNE or NNW or ESE, etc.  Thus, the errors here can be significant and open to interpretation.   In older materials (not to say it still does not occur in more recent documentation), you can find conflicting data all over the place.  I generally take this data with a grain of salt.  Even if it is "official" documents.  Data is only as good as some one makes it.  The data has to be accurately entered into the "official" records.  Humans make mistakes so if it does not look right to you, be suspicious.

For those airfields were no remains exist, coordinates have to be treated as estimates unless other data is available to support them.  Many simple fields used for airfields are the hardest to find remains, if any.