Underground Facility Mapping - Benefits Of GPS
Plan Characteristics
- Vary in age and quality
- Overlap with time as the facility infrastructure is expanded, upgraded and
repaired
- Plans redundantly show reference to facilities mapped on other plans
- Large number of plan sheets represent the service area
- Hand drawn updates maybe made on the plans
- Uncertain number of relevant plan sheets needed for conversion
- Contain very detailed definition of facility location and characteristics
- GIS relevant data is a small subset of all of the information shown
- Do not contain unique facility feature identification
- Plan details at a larger scale are used to show infrastructure detail
Atlas Mam
Atlas maps represent the utility infrastructure as a continuous map composed of
tiled map sheets. The atlas is intended to represent the current state of the network.
The maps generally do not represent utility features redundantly except at map
sheet edges where sheet-to-sheet overlap may be shown. The mapping symbology
and information content is usually consistent across the entire service area. Unique
facility feature identifiers may be shown on the maps to associate tabular
maintenance information.
The underlying basemap of the atlas maps maybe accurately mapped using
photogrammetric methods or precision coordinate geometry (COGO) methods.
Often however, the basemap may not be spatially accurate, having been developed
with hand drafting techniques not designed to result in a precision map.
Atlas Characteristics
- All utilities compiled consistently on a common map base
- Basemap maybe positionally inaccurate and maybe variable across the map
- The number of map sheets for the service area is known
- Facility locations may be a schematic to show facility components for visual
clarity
- If in CAD format, may not have enforced topology. Symbols or blocks maybe
used to represent complex features such as valve clusters.
- May lack engineering information detail needed for precision mapping
Map Conversion Methods
Digitizing
Map digitizing is usually the lowest cost method of inputting the utility
infrastructure into a GIS format U the source maps are sufficiently accurate to meet
the needs of the GIS. Digitizing directly from source documents will replicate
existing inaccuracies. Often the sources are not accurate enough and require re-drafting
onto an accurate base prior to digitizing. The re-drafting process is tedious,
time consuming, and may result in data of inconsistent and unknown positional
accuracy. The re-drafting process requires interpretation of utility locations onto a
new map base through scaling that is prone to judgment and process errors. The re-drafting
process is relatively fast, can be performed by low skilled technicians, and
much of it can be performed without a workstation. The resulting positional
accuracy confidence is moderate but often adequate for many needs.
COGO
COGO is a high precision method for inputting the location of facilities based on
drawing measurements. Facility location definitions found on record drawings
often reference street centerlines, easements, or other land base features. Because of
this, effective COGO development of a facility network requires a highly accurate
landbase with street centerlines, rights of way, and easements. The results achieved
from calculating the facility locations can be excellent when an accurate land base is
available.
Costs associated with COGO input are high compared to other techniques. The skill
level of the data entry operator must be high and a computer workstation is needed
for all phases of the work. Working from many engineering plans requires a
significant amount of document handling when trying to determine where each
sheet connects to other sheets. The engineering documents may only reflect the
network at the time of initial design or installation. Subsequent updates may not be
shown on engineering drawings, and if shown on atlas maps, they may lack
sufficient information to calculate the location of features with precision. Older
utility districts that have been developed and maintained over a long time frame
may have source documents that replace earlier documents. Careful sorting and
evaluation of documents prior to conversion adds to the effort further.
Photogrammetry
Photogrammetric mapping is an effective method of developing land based maps of
known accuracy. Orthophoto mapping or linear mapping of planimetric features
provide an excellent backdrop for many utility applications. Mapping underground
257.utility features with photogrammetric methods requires pre-marking surface facility
locations so they are visible on the aerial photography. Lines connecting the surface
features (valves, maintenance holes, hydrants, etc.) can be drawn quickly once the
features have been accurately located. This method canresult in very accurate
mapping, however there are several significant drawbacks.
Pre-marking of surface features is costly, time consuming, becomes “street graffiti”,
and irritates the public. Any facility components obscured by trees, vehicles or other
obstruction cannot be located. Aside from these significant problems, the mapping
can result in a highly accurate map of facilities. Information consistency between
the land base and utility may be excellent if both are developed from the same
photography.
Photogrammetry and GPS techniques provide methods of precise positioning of
features visible on the ground surface. Other features such as bends and crosses
must be mapped from as-builts or atlas drawings. The orientation of pipes and the
manner in which they connect to valves and hydrants must also be defined by the
atlas or as-built drawings.
GPS/Digitize
GPS provides a rapid means of field surveying utility locations. Real time
kinematic (RTK) positioning GPS technique enables centimeter-level positioning
instantly at the time of observation. This process of field surveying can replace
photogrammetric utility positioning and cost effectively locate every utility when
coupled with conventional survey techniques. Moreover, maintaining consistent
positioning of added or relocated facility components can easily be performed to a
consistent accuracy specification.
For initial data collection during facility conversion, the project must be laid out and
a plan developed for visiting every utility feature visible at ground surface. A GPS
base station is established, then mobile units carried in backpacks by surveyors are
carried to each valve, maintenance hole and other utility feature. The surveyor sets
up a prism pole with a GF’S antenna over each utility feature. Centimeter level
positioning can be achieved during one minute of GPS observation data. During
this time, the surveyor associates feature codes identifying the feature. Daily
production rates vary depending on the density of features. The time of point
occupation is considerably less than the time to pre-mark for aerial photography.
Facility positioning can be accomplished over any period of time with a consistent
accuracy of measurement. If cars are parked over a maintenance hole or valve, the
surveyor simply returns to the point at another time. By walking the field, all
258.visible utility features are measured. In many cases, utilities not shown on atlas
maps are located due to the 100% enumeration of facility features.
Conventional surveying must be used where structures obscure satellite visibility.
This may be a limiting factor in highly urbanized downtown areas that have tall
buildings. The RTK method requires continuous radio transmission between the
GPS base station and field unit. At times, interference or loss of signal can cause
complications and delays.
Mapping of the facility network is similar to the photogrammetric approach once
the surface features are surveyed. Digitizing is used to connect pipes between the
surveyed features guided by plans and atlas sheets. The reference land base must be
highly accurate so that the surveyed locations remain positioned relative to road
centerlines and other basemap features.
