Explanation of Survey Methods

September 4, 2009
Modified in 2013

Please E-mail comments, corrections and additions to the webmaster at pje@efgh.com.

Surveys are made on foot, road bike or mountain bike or in my truck. The survey notes usually indicate which method of transportation was used.

Surveys are dictated into a voice recorder at the time of the survey. This appears to be the only practical method of recording such information in the field. The recording is then transcribed, and the transcripts are archived. The recording medium is then erased and reused.

A few abbreviations are used in survey transcripts:

     DS  downstream          R   right
     E   east                S   south
     EB  eastbound           SA  straight ahead
     FR  fire road           SB  southbound
     L   left                SE  southeast
     N   north               SW  southwest
     NB  northbound          US  upstream
     NE  northeast           W   west
     NW  northwest           WB  westbound

     lat/lon  latitude and longitude
     sp       spelling

Matter added to the survey when the transcript is made is enclosed in brackets ([]). The notation "[sp]" means that the preceding name was spelled out in the dictation.

Interstate highways are represented in the usual way, by the letter "I" followed by a hyphen and the route number. Similarly, California, Arizona and U.S. Highways are represented by the letters "A", "C" and "U", respectively, followed by a hyphen and the route number. Some examples:

     I-8       Interstate Highway 8
     C-79      California Highway 79
     A-45      Arizona Highway 45
     U-395     U.S. Highway 395

Other abbreviations are used during dictation, but are normally expanded before the transcript is archived. However, I will occasionally forget to expand an abbreviation. One that might be puzzling if not expanded is "mba", which stands for "mid-block alley".

I have used two hand-held GPS units to determine the location of trails, streets and points of interest. Both of them measured latitude and longitude in degrees, minutes and fractional minutes (not seconds). The first one, which I used before September 12, 2008, measured to the nearest 1/100 minute. The current one, which I have used after September 12, 2008, displays to the nearest 1/1000 minute (although the actual measurement is probably not that precise).

I record the degrees only once, at the beginning of a survey. They usually don't change in the course of a survey, and it is quite obvious when they do change. Subsequent latitude and longitude measurements contain only the whole and fractional minutes.

Reliable recording of latitude and longitude measurements in the field requires a systematic approach. I record the following four integers in the order given:

  1. the whole number of minutes of latitude
  2. the number of hundredths of a minute (with the old GPS) or the number of thousandths of a minute (with the new GPS) of latitude
  3. the whole number of minutes of longitude
  4. the number of hundredths of a minute (with the old GPS) or the number of thousandths of a minute (with the new GPS) of longitude
The numbers are then embedded in the transcript, separated by hyphens.

Hence the coordinates N 32 43.305 W 117 4.792 would be recorded as 43-305-4-792. With the old GPS, the same coordinates would be recorded as 43-31-4-79.

An abbreviated form of the latitude and longitude is used when the reading is close to the previous one, as it would be when consecutive readings are taken along a trail or road. In this case, only the fractional parts of the latitude and longitude are recorded. In our example, this would be 305-792 or 31-79. To reconstruct the latitude and longitude of such a point, the whole numbers are taken from the previous reading, with adjustments up or down as necessary.

I have written a program to look through a transcript and plot all points represented by latitude and longitude measurements, connecting each pair of consecutive readings with a straight line segment. The program comes in two versions, depending on which GPS was used. In scaling the distances, it makes the assumption that the length of a minute of longitude or a minute of latitude is fixed over the area of the survey. For surveys of small areas, this is usually a reasonable approximation.

To make the plotted points intelligible, it is necessary to introduce two other features into the recorded coordinates, which are usually added when the notes are transcribed. An exclamation point (!) immediately after a latitude and longitude reading marks it as the end of a broken line. The plotting program draws a line segment from the previous reading to the marked one, but not from the marked one to the next one. A hyphen followed by text immediately after a reading indicates that the point is to be plotted in red, and the text is to be placed after it in the plot.

Occasionally, I have to eliminate a reading so the resulting line will make sense. I usually do this by using double hyphens instead of single hyphens to separate the two or four parts of the reading. This causes the plotting program to ignore it, but it retains the values in case I want to use them in the future. This technique is also used with hyphenated numbers that are not latitude and longitude readings.

I have learned that latitude and longitude readings are seldom repeatable exactly. The same trail junction will often give slightly different readings each time it is encountered. The plotting program will then produce a gap where there should be none. Therefore, I normally take such a reading only once and then insert bracketed copies of the first reading at subsequent encounters.

Finding latitudes and longitudes on standard USGS topographic maps is difficult, and most other maps don't have latitude and longitude. Therefore, the plotting program does not put its results directly on the map. It produces a .GIF image containing only its results on a white background. Examination of this image often shows the locations of blunders in the data. After the blunders have been corrected, the resulting image is laid over the map. I normally survey some features, such as streets, that appear on the map, so I can use them to position the image.

Photograph annotations use many of the same abbreviations as survey notes. I often take several pictures of the same scene, panning from left to right. Each one after the first is annotated as "R of #####", where "#####" is the number of the previous picture. Sometimes these can be stitched together, using a special image editing program.

2013 Addendum - New GPS

In early 2013, I acquired a new GPS with some enhanced features that have changed my survey procedure somewhat. It is a Garmin eTrex 30, and it has one feature that makes it well worth the $300+ price tag. I can connect the GPS to my computer and transfer data to the computer, without typing it in.

Therefore, in surveys taken with the new GPS, I record waypoint numbers instead of actual latitudes and longitudes. An equal sign is used to mark a waypoint: e.g., #23 is waypoint number 23.

An exclamation point following the waypoint number is used to mark the end of the track, as under the old system, e.g., #45! is the end of a track.

When the numbers of waypoints on a track are not consecutive, the plotting program automatically inserts the intervening waypoints.

An asterisk following a waypoint indicates a bad reading, e.g., =12* indicates that waypoint number 12 is to be ignored.

The GPS also allows tracking, which is the automatic recording of waypoints, regularly spaced by distance or time, or automatically.

When both waypoints and tracks are being recorded, they are kept in separate files.

Each waypoint and track point contains not only the latitude and longitude, but also the elevation and the date and time. For some strange reason, the date and time are UTC, which are the standard date and time on the prime meridian. I usually do not convert the date and time; sometimes I don't even preserve them in my records. Elevations are recorded in meters, with excessive precision. I convert them to feet and round them, although the expected error in an elevation is more than a single foot.

Tracks are most useful when points of interest are annotated. It is not feasible to do this directly. However, annotated waypoints taken during tracking can be merged with the track points by simply sorting all the points by date and time.

Garmin sells software to handle waypoints and tracks, but I prefer to use my own. I save waypoints and track points in database files of the dBase III or .DBF format, which is readily read and manipulated by most database programs. The fields are as follows:

SERIAL Numeric 4 0 Applies only to waypoints. For tracks it is meaningless, and is usually set to zero. Width is only 3 in some smaller tables.
LATITUDE Numeric 11 6 In degrees and fractions of a degree north of the equator. It is always positive.
LONGITUDE Numeric 11 6 In degrees and fractions of a degree west of the prime meridian. It is always negative.
ELEVATION Numeric 5 0 In feet above sea level.
DISTANCE Numeric 6 2 Normally left blank. A special program is used to compute the distance, in miles, from the first point, and insert it into this field.
DATE Date 8   Date of the UTC timestamp. Omitted from some archived data.
TIME Numeric 6   A six-digit number of the form HHMMSS giving the hour (on a 24-hour clock), minute and second of the UTC timestamp. Omitted from some archived data.
COMMENT Character 64   Applies only to waypoints. It is filled in manually from recorded field notes.

Philip J. Erdelsky