5.3 Control Networks and Datum Selection
Key Takeaways
- Choose one horizontal datum and one vertical datum for the job and write zone, epoch, units, azimuth basis, combined factor, and benchmarks on the plans.
- Control hierarchy is published → project → working; construction stakes do not redefine the datum, and the design datum is held through construction.
- Assumed-to-CCS83 translation example: ΔN = 2,082,452.18 ft and ΔE = 6,041,883.40 ft, with 0.02 ft residuals at a second monument 703.66 ft away.
- Reporting as-built elevations on NGVD 29 or as ellipsoid heights when the plans are NAVD 88 is a vertical-datum fail, not a rod fail.
- Changing combined factor from 0.999935 to 1.000000 mid-job makes a 2,400.00 ft grid street 0.16 ft short if it is then staked as 2,400.00 ft on the ground.
5.3 Control Networks and Datum Selection
Quick Answer: Pick one horizontal datum and one vertical datum for the job, write them on the plans, and hold them through construction. Organize control as published → project → working. When a city requires CCS83 deliverables from an assumed field system, translate with a check on a second monument. Classic failures: elevations reported on a different vertical datum than the plans, and a combined factor that changes in the middle of the job.
Choose Datums, Then Write Them Down
A civil surveying job does not need every datum that exists. It needs one horizontal and one vertical system that everyone—designer, checker, inspector, contractor—can name. Domain I.F is a Survey Planning item (part of the 25% planning weight) because every later traverse, level loop, map, and stake inherits that choice.
Horizontal examples:
- Assumed: origin Hub A N = 10,000.00 ft, E = 20,000.00 ft; project north along A–B.
- Mapped: CCS83 Zone 3 (2011) epoch 2010.00, U.S. survey feet.
Vertical examples:
- NAVD 88, BM-City = 247.32 ft, project BM-P1 = 254.11 ft.
- Assumed: BM-P1 = 100.00 ft.
The two choices are independent. You may hold assumed horizontal with NAVD 88 vertical. That split is professional only if the title sheet says so. Silence is how a hydrology report in NAVD 88 meets a grading plan in assumed 100.00 and nobody notices until the detention invert is a foot off.
What to put on the plans (control diagram or general notes):
| Element | Example note |
|---|---|
| Horizontal system | Assumed, or CCS83 Zone 5 (2011) epoch 2010.00, U.S. survey feet |
| Origin / monuments | Hub A N/E; ties to CITY-A and CITY-B |
| Azimuth basis | Project north along warehouse face A–B, or grid north |
| Combined factor (if mapped) | 0.999932, ground to grid, held for the contract |
| Vertical system | NAVD 88 |
| Benchmarks | BM-City 247.32; BM-P1 254.11; date of level loop |
| Conversion (if any) | NAVD 88 = NGVD 29 + 2.47 ft at this site (city note) |
If the job later needs NATRF2022 or CCS2022, that is a new named system, documented the same way—not a silent GNSS firmware default. Until the plans say so, hold what the plans name.
Hierarchy: Published, Project, Working
Published control is the named network: NGS, the California Spatial Reference Center / California Spatial Reference Network (CSRC / CSRN), Caltrans, city, or county monuments and benchmarks. You recover it; you do not invent published coordinates.
Project control is the primary on-site network on the chosen datums: closed traverse, closed level loop, monumented to last through design and construction. Chapter 3 taught why that skeleton exists. This section adds the datum rule: project control is the job datum, not a pile of extra hubs with mystery numbers.
Working control is daily occupancy: a nail for this week's curb, a hub for a catch-basin layout. It is checked back into project control. It does not redefine CCS83, and it does not redefine NAVD 88.
Construction stakes remain products of control, not sources of it. If a primary monument is destroyed, you re-establish from remaining project or published points—not from a chewed-up offset lath whose elevation was a finished-grade call.
A numeric picture of the hierarchy: two published city monuments 703.66 ft apart fix the mapped datum (worked example below). Three primary project hubs occupy that system and close a traverse. Twenty working nails come and go. If a working nail disagrees with a primary hub by 0.15 ft, you throw out the nail, not the hub. If a primary hub disagrees with both published monuments by 0.15 ft, you stop and find the bust before you stake a mile of curb.
Hold the Design Datum Through Construction
The contractor stakes the coordinates and elevations that are on the permitted plans. If those plans are assumed N/E with NAVD 88 elevations, the field crew does not "upgrade" to CCS83 on Monday because a GNSS base made it easy—unless the entire design surface, the alignment stations, the quantity files, and the inspector's checks are transformed together and the change is issued as a plan revision.
Holding the datum includes holding the combined factor and the azimuth basis. A localization that silently sets scale = 1.000000 on a job whose design used 0.999932 is a new horizontal datum wearing the old coordinate numbers. A crew that orients to true north while the plans still hold warehouse-face project north repeats the 74.7 ft rotation bust from Section 5.1, now during construction instead of during design.
Translating Assumed to CCS83
Cities often require digital deliverables on CCS83 even when the most efficient field system was assumed. The professional sequence is: survey in one system, transform, check, then submit. Do not leave the city with assumed 5,000/10,000 numbers labeled as Zone 3.
Worked example: two-monument translation
Two city monuments are recovered. CCS83 Zone 3 (2011) epoch 2010.00, U.S. survey feet:
| Point | Assumed N (ft) | Assumed E (ft) | CCS83 N (ft) | CCS83 E (ft) |
|---|---|---|---|---|
| CITY-A | 5,000.00 | 10,000.00 | 2,087,452.18 | 6,051,883.40 |
| CITY-B | 5,466.50 | 10,526.80 | 2,087,918.66 | 6,052,410.22 |
Translation using CITY-A as the origin of the shift:
ΔN = 2,087,452.18 − 5,000.00 = 2,082,452.18 ft
ΔE = 6,051,883.40 − 10,000.00 = 6,041,883.40 ft
Check on CITY-B:
Assumed N + ΔN = 5,466.50 + 2,082,452.18 = 2,087,918.68 ft (published 2,087,918.66; residual +0.02 ft)
Assumed E + ΔE = 10,526.80 + 6,041,883.40 = 6,052,410.20 ft (published 6,052,410.22; residual −0.02 ft)
Distance CITY-A to CITY-B on the assumed system: √(466.50² + 526.80²) = √(217,622.25 + 277,518.24) = √495,140.49 = 703.66 ft. The two-point residual is a couple of hundredths over that line. A pure translation is acceptable at this site. If residuals were tenths or feet, you would solve a rotation (site north versus grid north) and possibly a scale (combined factor), then still check a third point. Do not force a one-point shift when the second monument disagrees.
Design point P, assumed N = 5,210.00, E = 10,340.00:
N_CCS83 = 5,210.00 + 2,082,452.18 = 2,087,662.18 ft
E_CCS83 = 10,340.00 + 6,041,883.40 = 6,052,223.40 ft
Submit P with the zone, epoch, and unit. The assumed coordinates remain the construction system if the plans still hold assumed; the CCS83 values are the mapped deliverable. If construction must also move to CCS83, issue that as a single, complete revision—not a field-book-only change. Adding the assumed 5,210.00 onto the already-translated CITY-A coordinates a second time (a common spreadsheet error) would throw P about 5,000 ft north of the site.
Exam trap: elevations on a different vertical datum than the plans
The grading plan, the hydraulic profile, and the construction stakes must share a vertical datum. A surveyor who reports as-built rims as NGVD 29 because that is what the 1988 city list said, while the 2026 plans are NAVD 88, manufactures a 2-ft class error without touching the rod (2.47 ft in the Section 5.2 site note). The same trap: GNSS-derived ellipsoid heights in an as-built table headed "NAVD 88," which can be tens of feet off. Match the plan note, not the most convenient file in the truck.
If the inspector's daily reports use assumed 100.00 while the engineer of record stamps NAVD 88, every "cut 0.15" instruction is being applied to the wrong zero. Fix the paperwork before you fix the dirt.
Exam trap: changing the combined factor mid-job
Design and staking used combined factor 0.999935 (grid = ground × 0.999935). A GNSS localization later in construction uses scale 1.000000.
A 2,400.00 ft grid street length on the plans corresponds to ground = 2,400.00 / 0.999935 = 2,400.16 ft.
If the new localization stakes 2,400.00 ft on the ground for that same grid dimension, the layout is 0.16 ft short. Per 800 ft of street the error is 800 × (1/0.999935 − 1) = 0.052 ft; at 1,600 ft it is 0.104 ft; at 3,200 ft it is 0.208 ft. None of that is random EDM noise. It is a datum/scale change that was never written on the plans.
Other mid-job scale tricks: mixing grid stationing with ground-distance pay quantities without a note; applying one combined factor on the west half of a corridor and a freshly interpolated factor on the east half without a revision. If the factor must change because the job truly spans a large elevation or zone-scale gradient, document a new factor, transform the design, and issue it. Do not let the data collector pick it.
Putting Domain I.F together
Horizontal and vertical datums, including assumed non-geodetic systems, are a Survey Planning knowledge item because every later measurement inherits them. Section 5.1: pick and hold a horizontal system; know CCS83 at a zone-and-factor level. Section 5.2: pick and hold a vertical system; close the level loop. This section: combine those choices into a control network, document it, transform when a mapped deliverable is required, and refuse silent mid-job changes.
On the exam, when a stem gives two elevations, two northings, or a Google overlay, ask which named system each number belongs to before you add, subtract, or stake.
Which statement correctly describes control hierarchy and datum selection on a civil job?
CITY-A is assumed N = 5,000.00 ft, E = 10,000.00 ft and CCS83 N = 2,087,452.18 ft, E = 6,051,883.40 ft. Using that translation, what are the CCS83 coordinates of design point P at assumed N = 5,210.00 ft, E = 10,340.00 ft?
Design used combined factor 0.999935. A 2,400.00 ft grid street is later staked with a localization that uses combined factor 1.000000, laying out 2,400.00 ft on the ground. What happened?