The geodetic marks in the NGS database are adjusted. The horizontal control marks are horizontally adjusted and vertically scaled, while the vertical control marks are vertically adjusted and horizontally scaled. A few marks are both horizontally and vertically adjusted. Adjusted is a term used to indicate that a computer program has analyzed a set of survey data and recaluclated the positions of all the points in the survey. The recalculation process is called adjustment.

When you go searching for a geodetic mark, it is very important to note whether the mark is horizontally adjusted or horizontally scaled.

If the mark is horizontally adjusted, its adjusted horizontal coordinates will be rounded to 5 decimal places in the format dddmmss.sssss and accurate to that level. This is much more accurate than can be achieved by your handheld GPS receiver which has a precision of dddmmss.s and a level of accuracy corresponding to the single decimal place.

If the mark is horizontally scaled, its horizontal coordinates can be up to a few hundred feet off, since, instead of being accurately measured and adjusted, they were probably scaled from a topographic map, perhaps even a 15-minute quad and not adjusted.

The concept of adjustment is an interesting mix of statistics, calculus, trigonometry, and matrix algebra. Imagine that you are to survey along all the streets in a suburban area. The process of measuring with instruments has natural error in the process, both systematic error (hopefully little to none of that!) and statistical error. You survey up and down all the streets, establishing a survey station at each intersection and other stations between intersections as well. When you finish and go back to calculate results, no matter where you begin and calculate the coordinates of each intersection station, you will find that, at intersection stations you have calculated coordinates for before, you have now calculated a second different set of coordinates for the same survey station. If you start at a different point, you get different results. What you have is a matrix of 'opinions' of where each intersection is. The solution to this dilemma is to use an adjustment program. It simuiltaneously calculates the location coordinates of every intersection in the matrix to take into account all the 'opinions'. It uses what is called a least-squares best fit, a statistical concept that relates to the fact that statistical error grows like a square-root function (a curve). It thereby adjusts the coordinates of all the intersection stations for the best mathematical fit and then goes on to adjust the locations of all the stations between intersections.

An adjustment program uses statistical calculations to identify any survey links that have an out-of-tolerance anomalous error and flag them, thereby protecting a final solution from obvious errors in the data. An adjustment program can also use statistical mathematics to calculate the precision of the values of the adjusted coordinates for the appropriate rounding level.