Surveying relies on physical reference objects placed in the ground. While temporary stakes are what the public may see most often in conjunction with construction work, the enduring reference marks (usually called monuments) are much less prominent. Monuments serve to mark points used for geodetic control networks as well as points used to reference property boundaries. Monuments can take a variety of forms.
The location of a monument may be recorded directly on a map, and nearby objects may also be documented to aid in finding (“recovering”) a monument at a later date. A map showing a monument with distances and directions to surrounding objects is called a “tie sheet”. Photographs may be used to further assist in recovering a monument, and mapping coordinates (even if of low accuracy) may be provided so that a GPS receiver can be used to navigate to the vicinity.
A monument may be labeled with the name of the organization that set it and other useful identifying information. A more visible nearby post (“witness post”) may also be labeled.
Categories of monuments
Terminology describing monuments derives from their purposes or method of
establishment. Older horizontal geodetic control monuments are often called
triangulation stations (“trig stations”) while newer ones may be
somewhat casually called GPS stations. A vertical control station set in the
ground is called a benchmark (“BM”), and a place where a point
on an existing stable object (e.g., a ledge of a bridge abutment or building’s
concrete foundation) is used is called a useful elevation (UE). Monuments along
the boundary of a land parcel are commonly placed at corners, points of major
deflection in the boundary.
Materials and installation
The ideal monument is sturdy enough to persist in the landscape for decades
by resisting forces that would corrode, erode, move, or otherwise disrupt
it. However, setting monuments involves costs, so a range of monument types
have been used over the years in various settings.
The depth to which a monument is set is an important consideration. In cold climates such as Wisconsin’s, the annual freeze-thaw cycle in the ground can cause movement over time in monuments that are not set deeply. This stability issue is critical for geodetic control monuments, particularly those for which vertical (elevation) values are published.
A common modern style is a stainless steel pipe or rod driven into the ground (sometimes to “refusal”, i.e., as deep as the driving equipment can achieve), set in concrete, and affixed with a survey cap. The cap is typically stamped with the name of the agency responsible for the monument and other information specific to that individual survey station. Sometimes an auger is used to form the cavity into which concrete is poured.
Land parcel corners typically are monumented with a less robust approach. At the simplest, iron pipe or concrete reinforcing bar (“rebar”) may simply be pounded into the ground. Historically, the original government land surveyors set wooden PLSS corner posts except in unforested areas where charcoal was to be buried beneath a pile of stones. Since a large number of PLSS corners are in roadways, the monument may be below the pavement and recoverable through use of a metal detector and/or through ties to nearby objects; a separate monument may be offset nearer to the edge of the right of way, with distance and direction to the actual corner provided for users.
Location of monuments
A surveyor may or may not have flexibility in where to place a monument. For
a property corner (including the PLSS), there is usually only one choice.
For geodetic control, on the other hand, the location is chosen to meet practical
concerns about line of sight, spacing between nearby monuments, and/or ease
of access.
Disturbance/Destruction
The value of control information or boundary marking is jeopardized by any
disturbance to a monument, so protection is key. Disturbing a federal survey
marker is a misdemeanor and carries a $250 fine for deliberate tampering.
Missing or disturbed geodetic control markers, as well as those in danger
of being disturbed, can be reported to the agency stamped on the cap or by
filling out a condition report
form available
online from the National
Geodetic Survey. The county surveyor or
land information office is the
best place to begin when reporting a problem with a PLSS monument.
Note that uninformed attempts to restore a disturbed monument can be extremely
counterproductive:
If a geodetic control monument is disturbed, yet evidence of that disturbance is concealed, users of the monument will not know that its published position and/or elevation are no longer correct. Similarly, a property boundary monument that has been moved will provide a false impression of correct location.