Differential Leveling
Overview
Differential
leveling is the process of measuring vertical distances from a known elevation
point to determine elevations of unknown points. The most common methods to
determine elevation are through the use of 1) a compensator type, automatic
(engineering level) and level rod(s), and 2) an electronic digital barcode
leveling instrument with barcode rod. A thorough knowledge of leveling
principles and proper application of methods and equipment will prevent costly
delays and generate the needed results and accuracy.
leveling is the process of measuring vertical distances from a known elevation
point to determine elevations of unknown points. The most common methods to
determine elevation are through the use of 1) a compensator type, automatic
(engineering level) and level rod(s), and 2) an electronic digital barcode
leveling instrument with barcode rod. A thorough knowledge of leveling
principles and proper application of methods and equipment will prevent costly
delays and generate the needed results and accuracy.
Preferred
methods of obtaining elevations (in order of preference/accuracy) are:
methods of obtaining elevations (in order of preference/accuracy) are:
·
Differential Leveling – control bench marks, cross-sections, point
elevations
Differential Leveling – control bench marks, cross-sections, point
elevations
·
Trigonometric Leveling – for Digital Terrain Model (DTM), 3D Model
Trigonometric Leveling – for Digital Terrain Model (DTM), 3D Model
·
Indirect leveling (and location) by measuring horizontal distances
and vertical angles
Indirect leveling (and location) by measuring horizontal distances
and vertical angles
·
Three-dimensional coordinates – both horizontal position and
elevation are computed by processing the measurements
Three-dimensional coordinates – both horizontal position and
elevation are computed by processing the measurements
·
GPS survey – given the appropriate equipment, procedure and data
collection.
GPS survey – given the appropriate equipment, procedure and data
collection.
Equipment
·
The Rod
The Rod
o Rods are, in essence, a
tape supported vertically, and used to measure the vertical distance
(difference of elevation) between a horizontal line of sight and a required
point above or below it. The most common types are the Philadelphia rod- a 2
piece rod usually 13′ in length, the Frisco rod- a 3 piece rod 12′-15′ in
length, the Lenker rod- a moveable face, direct reading rod, and the
fiberglass-rod- a multi-section, extension rod from 8′ – 45′ in length.
Electronic digital levels use a barcode marked rod. Precision leveling, known
as First Order leveling, to extend or establish vertical control over long
distances, requires use of invar scale rods.
tape supported vertically, and used to measure the vertical distance
(difference of elevation) between a horizontal line of sight and a required
point above or below it. The most common types are the Philadelphia rod- a 2
piece rod usually 13′ in length, the Frisco rod- a 3 piece rod 12′-15′ in
length, the Lenker rod- a moveable face, direct reading rod, and the
fiberglass-rod- a multi-section, extension rod from 8′ – 45′ in length.
Electronic digital levels use a barcode marked rod. Precision leveling, known
as First Order leveling, to extend or establish vertical control over long
distances, requires use of invar scale rods.
o All rods should be checked
and maintained to ensure consistent readings. Cleaning and adjustment or repair
should be done as needed.
and maintained to ensure consistent readings. Cleaning and adjustment or repair
should be done as needed.
·
The Level
The Level
o The compensator type,
automatic (engineer) level is made by various manufacturers, and is a
precision, self-leveling instrument, equipped with a built-in compensator that
automatically maintains a horizontal line of sight and has a telescope with
approximately 30-power magnification. The level mounts on a standard surveying
tripod, or a fixed-leg tripod for more precise leveling.
automatic (engineer) level is made by various manufacturers, and is a
precision, self-leveling instrument, equipped with a built-in compensator that
automatically maintains a horizontal line of sight and has a telescope with
approximately 30-power magnification. The level mounts on a standard surveying
tripod, or a fixed-leg tripod for more precise leveling.
o The electronic digital
level is also a precision, self-leveling instrument with additional advantages.
Advanced capabilities include automatic measurement of height and distance by
reading a barcode rod, high precision by employing image processing technology,
data display and data recording either internally or to a data collector,
installed software for elevation stake-out or other leveling procedures. The
digital level offers greater productivity and simplicity in all applications of
leveling work.
level is also a precision, self-leveling instrument with additional advantages.
Advanced capabilities include automatic measurement of height and distance by
reading a barcode rod, high precision by employing image processing technology,
data display and data recording either internally or to a data collector,
installed software for elevation stake-out or other leveling procedures. The
digital level offers greater productivity and simplicity in all applications of
leveling work.
o All leveling equipment
must be checked regularly and properly maintained to ensure that it remains in
proper working condition and that systematic errors are eliminated to produce
the expected results. This includes daily checks, periodic routine maintenance
and yearly cleaning and adjustment by a qualified repair shop.
must be checked regularly and properly maintained to ensure that it remains in
proper working condition and that systematic errors are eliminated to produce
the expected results. This includes daily checks, periodic routine maintenance
and yearly cleaning and adjustment by a qualified repair shop.
o Daily – keep the
instrument clean by wiping occasionally, especially when used in dusty or wet
environment. Operate and adjust the motions. With a compensator type, automatic
level, observe the compensator while adjusting level screws to make sure that
motion is smooth. If erratic compensator movement is detected, have a qualified
repair shop service the instrument.
instrument clean by wiping occasionally, especially when used in dusty or wet
environment. Operate and adjust the motions. With a compensator type, automatic
level, observe the compensator while adjusting level screws to make sure that
motion is smooth. If erratic compensator movement is detected, have a qualified
repair shop service the instrument.
Instrument
Check
On a
regular basis and before beginning a leveling project, perform a peg test for
collimation error to make sure that readings are consistent. If not, have the
instrument serviced.
regular basis and before beginning a leveling project, perform a peg test for
collimation error to make sure that readings are consistent. If not, have the
instrument serviced.
Anytime
an instrument is dropped or severely bumped, or suspected of the same, it
should be checked immediately. If questionable, have the instrument serviced
before continuing use.
an instrument is dropped or severely bumped, or suspected of the same, it
should be checked immediately. If questionable, have the instrument serviced
before continuing use.
Leveling
Methods
·
2nd Order for primary elevation control, extending or establishing
bench marks on a project.
2nd Order for primary elevation control, extending or establishing
bench marks on a project.
·
3rd Order intermediate or temporary bench marks, turning points on
a project; aerial photo mapping control; major structures.
3rd Order intermediate or temporary bench marks, turning points on
a project; aerial photo mapping control; major structures.
·
General data collected for a DTM, topographic mapping, cross
sections, or other purpose.
General data collected for a DTM, topographic mapping, cross
sections, or other purpose.
Leveling
Tolerance
Datum
The NAVD
88 vertical datum shall be used on all TxDOT projects unless specified by TxDOT
prior to the beginning of the survey.
88 vertical datum shall be used on all TxDOT projects unless specified by TxDOT
prior to the beginning of the survey.
Three-Wire
Leveling
Three-wire
leveling is the process of reading and recording a rod reading for each of the
three horizontal cross-hairs on each shot, then averaging the readings for
agreement with the center reading. This method is most accurate as it gives
immediate results and a check/confirmation of rod reading. If a difference is
detected a check can be done on the spot, before moving on.
leveling is the process of reading and recording a rod reading for each of the
three horizontal cross-hairs on each shot, then averaging the readings for
agreement with the center reading. This method is most accurate as it gives
immediate results and a check/confirmation of rod reading. If a difference is
detected a check can be done on the spot, before moving on.
An
electronic digital level can perform and record this process automatically,
after sight of the rod is made. Automated data collection eliminates
transposition errors and data is downloaded into a computer for complete
analysis.
electronic digital level can perform and record this process automatically,
after sight of the rod is made. Automated data collection eliminates
transposition errors and data is downloaded into a computer for complete
analysis.
This is
the preferred method for setting or checking control bench marks. Three-wire
leveling will be used for all orders of leveling except general.
the preferred method for setting or checking control bench marks. Three-wire
leveling will be used for all orders of leveling except general.
Bench Marks
(BM) and Temporary Bench Marks (TBM)
The
primary purpose of running a level loop is to establish points of known
elevation or bench marks. They are solid, well protected points that can be
relied upon to remain undisturbed and unchanged. They should be positioned so
as to be usable from a wide area and away from the construction as much a
possible so as to remain undisturbed. Consideration should be given to
construction activities such as utility re-location proposed within the ROW. It
is preferred that bench marks are located in a public ROW, to allow for
continued accessibility.
primary purpose of running a level loop is to establish points of known
elevation or bench marks. They are solid, well protected points that can be
relied upon to remain undisturbed and unchanged. They should be positioned so
as to be usable from a wide area and away from the construction as much a
possible so as to remain undisturbed. Consideration should be given to
construction activities such as utility re-location proposed within the ROW. It
is preferred that bench marks are located in a public ROW, to allow for
continued accessibility.
The exact
location of the bench mark should be carefully selected, then sketched and
recorded in notes, a field book or a data collector.
location of the bench mark should be carefully selected, then sketched and
recorded in notes, a field book or a data collector.
The
numbering or identification system should be consistent with district numbering
convention, if any and should be identified with enough detail for another crew
to locate the bench mark easily. Bench marks set should always be turned
through as part of a closed level loop.
numbering or identification system should be consistent with district numbering
convention, if any and should be identified with enough detail for another crew
to locate the bench mark easily. Bench marks set should always be turned
through as part of a closed level loop.
Examples
of good bench marks are an iron rod driven to refusal or a TxDOT cap set
vertically in a concrete monument (similar to a Type II ROW Monument), a ROW
monument, a point or corner of a stable concrete structure, or occasionally a
spike set in a large tree or utility pole. It is usually advisable to use a variety
of types of bench mark monuments as utility relocation or construction activity
may remove objects from a ROW.
of good bench marks are an iron rod driven to refusal or a TxDOT cap set
vertically in a concrete monument (similar to a Type II ROW Monument), a ROW
monument, a point or corner of a stable concrete structure, or occasionally a
spike set in a large tree or utility pole. It is usually advisable to use a variety
of types of bench mark monuments as utility relocation or construction activity
may remove objects from a ROW.
The bench
marks should not include objects subject to tampering or removal.
marks should not include objects subject to tampering or removal.
Two or
more bench marks should be used from the specified reference datum (NAVD 88
unless directed by TxDOT). These bench marks should be identified, confirmed
undisturbed, and elevations proved by running a complete level loop between the
two, returning to the starting bench mark.
more bench marks should be used from the specified reference datum (NAVD 88
unless directed by TxDOT). These bench marks should be identified, confirmed
undisturbed, and elevations proved by running a complete level loop between the
two, returning to the starting bench mark.
Distance
between bench marks should be confirmed with the TxDOT project manager or the
district survey coordinator before beginning a project.
between bench marks should be confirmed with the TxDOT project manager or the
district survey coordinator before beginning a project.
Turning
Points/Temporary Bench Marks (TP/TBM)
Turning
points may be points set either before or during the course of the survey, or
natural or man-made points in the area. They must be solid, well defined (or
marked) and permanent enough to remain intact until the level loop is finished.
Points with a small, sharply defined top are preferred to large flat surfaces.
points may be points set either before or during the course of the survey, or
natural or man-made points in the area. They must be solid, well defined (or
marked) and permanent enough to remain intact until the level loop is finished.
Points with a small, sharply defined top are preferred to large flat surfaces.
Turning
points should be marked when used so as to insure that the rod is in the exact
same place when the backsight and foresight are taken. They are also marked
because turning points that are part of a closed level loop are points of known
elevation that may have value during future surveys in the area.
points should be marked when used so as to insure that the rod is in the exact
same place when the backsight and foresight are taken. They are also marked
because turning points that are part of a closed level loop are points of known
elevation that may have value during future surveys in the area.
Temporary
bench marks can be turning points that remain or additional intermediate
bench marks placed for added convenience.
bench marks can be turning points that remain or additional intermediate
bench marks placed for added convenience.
Temporary
bench marks set in trees or power poles should consist of a spike (railroad
spike, boat spike, or large nail spike) set horizontally approximately 1 to 2
ft. above ground elevation, also free of above obstructions to the level rod.
bench marks set in trees or power poles should consist of a spike (railroad
spike, boat spike, or large nail spike) set horizontally approximately 1 to 2
ft. above ground elevation, also free of above obstructions to the level rod.
General Considerations
/ Objectives
During
the course of running a level loop, choose/set turning points and bench marks
to accomplish the required objective and accuracy.
the course of running a level loop, choose/set turning points and bench marks
to accomplish the required objective and accuracy.
Balance shot distances – The rodman and instrument man must work as
a team to balance the backsight and foresight distances. This can be
accomplished by use of a digital level, estimating distance by pacing,
three-wire stadia difference or, when available, by observing stationing marked
on the project. Balanced backsights and foresights, essential in precise
leveling, will help eliminate errors caused by refraction, the curvature of the
earth and an instrument that is out of adjustment and are an especially
necessary procedure when establishing control bench marks.
a team to balance the backsight and foresight distances. This can be
accomplished by use of a digital level, estimating distance by pacing,
three-wire stadia difference or, when available, by observing stationing marked
on the project. Balanced backsights and foresights, essential in precise
leveling, will help eliminate errors caused by refraction, the curvature of the
earth and an instrument that is out of adjustment and are an especially
necessary procedure when establishing control bench marks.
Maximum sight distance – care should always be given to observe
recommended or required distance of sight, depending on the purpose of
leveling. See the NGSspecifications and Table
4.3 TSPS Manual of Practice
Chart for Tolerances for Conditions, found in this chapter, or the
manufacturer’s recommendations.
recommended or required distance of sight, depending on the purpose of
leveling. See the NGSspecifications and Table
4.3 TSPS Manual of Practice
Chart for Tolerances for Conditions, found in this chapter, or the
manufacturer’s recommendations.
Control
points and bench marks should always be set to the highest level of accuracy
suitable for the project or a higher level if it can be justified.
points and bench marks should always be set to the highest level of accuracy
suitable for the project or a higher level if it can be justified.
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