Most builder-grade automatic levels graduate the horizontal circle in 360 degrees with 1-degree increments for layout and angle-measurement work, while metric-region automatic levels offer the same circle graduated in 400 gon with sub-gon resolution [S1][S2][S5].
The horizontal circle is the rotating graduated ring around the base of an automatic level; its only job is to let the operator read a horizontal angle between two sightings without resorting to a theodolite [S2].
What "360 Degree" and "400 Gon" Actually Mean on the Dial
Sexagesimal (360°) graduation splits a full rotation into 360 parts, with each degree further divisible into 60 minutes; the SitePro 24X, 28X, and 32X auto levels all carry this 360° horizontal circle in 1-degree increments with stated angular accuracy of 1° [S1]. Gon (also called grad or grade) splits a full rotation into 400 parts, where a right angle equals exactly 100 gon, so 1 gon equals 0.9 angular degrees, and the typical sub-division is 0.5 gon or 1 gon, giving a direct decimal fraction of a right angle [S5]. South's ALTAS datasheet explicitly lists "Circle Graduation 360 degrees or 400 gon" as a user-selectable option on the same instrument, confirming both conventions coexist in the same product class [S5].
The conversion is mechanical, not magical: 360° = 400 gon, therefore 1° = 1.111 gon, 1 gon = 0.9° = 0°54', and 0.01 gon = 0.009° = 32.4" [S5]. Because the circle's only role in an auto level is coarse angle reading (fine leveling comes from the compensator, not the circle), the choice of unit is driven by the project datum and crew habit, not by inherent precision.
Selection Criteria: Region, Datum, and Crew Workflow
Pick 360° when the job site uses imperial/US customary units, when the chain of record ties to state-plane or U.S. public-land coordinates expressed in DMS, and when the rod reader and instrument operator are trained on one-degree layouts; Johnson Level's reference instrument description matches this convention directly, listing the horizontal circle as "marked at each 1 degree and has measurements up to 360 degrees" [S2]. Pick 400 gon when the contract is metric, the deliverable is a cadastral or topographic map in grads, or the workflow uses decimal bearings because 100-gon right angles and 50-gon offsets eliminate minute-second arithmetic in field books.
Three concrete decision criteria drive the choice. First, angular resolution at the circle: 360° units typically print 1° divisions (whole-degree tick marks), so the smallest readable increment is 1° = 1.111 gon, which is adequate for general construction layout [S1][S2]. Second, ergonomics: a 400-gon dial with 0.5-gon graduations gives roughly 0.45° resolution, slightly better than a 1° dial, but only if the reticle index is sharp enough to interpolate. Third, interoperability with downstream instruments: a 360° auto level pairs naturally with a total station reporting DMS angles, while a 400-gon auto level pairs with metric total stations and European mapping software; mixing the two in one traverse requires conversion at every book entry.
Comparison: 360° vs 400 Gon on the Same Instrument Family
South's ALTAS datasheet is the cleanest published spec sheet showing both options on one body, so the comparison can be drawn on equal hardware [S5]. Both versions share a magnetic-damping compensator with ±15' working range and ±0.6" setting accuracy, an 8'/2mm bubble sensitivity, IP54 ingress protection, and a -20°C to +50°C working envelope [S5]. Magnification steps 20X/24X/28X/32X with 34–38 mm objective apertures, and 1 km double-run accuracy improves from 2.5 mm (20X) to 1.0 mm (32X); the circle graduation choice does not affect these optics or accuracy figures [S5]. SitePro's 24X/28X/32X family likewise ships only in 360° with angular accuracy stated at 1° and leveling accuracy from ±1.5 mm @ 45 m (24X) to ±1.5 mm @ 76 m (32X) [S1].
The decision matrix therefore reduces to four criteria: region (US/Canada/UK generally 360°, most of Europe, Latin America cadastral jobs, and China generally 400 gon), unit system (imperial feet-tenths-hundreds reads naturally on 1° ticks; metric meters on 0.5-gon ticks), downstream coordinate format (DMS plan sheets want 360°; decimal grads/gon maps want 400 gon), and crew training (existing field-book templates lock the choice).
Who the 360° Dial Is For, and Who Should Skip It
The 360° horizontal circle fits general contractors, building layout crews, and road-grading teams working in imperial units or DMS deliverables, where angle readings only need to identify cardinal and oblique building lines to the nearest degree [S1][S2]. For most construction layout this is enough: the operator rotates the telescope until the index marks 90°, 180°, 270°, 360° at the four cardinal checkpoints during the initial leveling procedure, confirming the bubble returns to center at every quadrant [S3][S4]. The Johnson Level reference notes that once the bubble in the circular (bulls-eye) level is centered, the compensator takes over and the horizontal circle is only used to set and read horizontal angles, not to maintain level [S2].
Skip the 360° auto-level circle for any task requiring sub-degree angle work, for cadastral surveys delivered in grads, or for monitoring networks that will be reduced in decimal degrees; an automatic level horizontal circle, whether 360° or 400 gon, is a coarse angle-reading tool, and any job needing better than 1° angular accuracy should move to a theodolite or total station [S2].
Real Use Cases and Setup Procedure
Typical field deployment is layout of footing corners, establishing drainage slopes, setting suspended-ceiling heights, and verifying elevations of foundations and walls, all of which need horizontal angles only to the nearest 1° because the dominant errors are vertical (rod reading, collimation) rather than horizontal (angle reading) [S2]. For the rough leveling that prepares the circle for use, the civilgadgets reference walks the standard three-screw routine: place the telescope parallel to two foot screws, bring the bubble into the circle by counter-rotating the pair, adjust the third screw to center, then rotate 90° and confirm the bubble stays centered, repeating through 180°, 270°, and 360° to lock in a true horizontal plane before any angle is read [S3][S4].
On the 400-gon side, the ALTAS package ships with an aluminum 5-section staff graduated on dual sides in large E and millimeter scale, 5 m extended length, ±0.5 mm/m linearity and ±3 mm overall deviation, with a 5/8" thread flat-head twist-lock tripod, extended height 1600 mm and closed 970 mm [S5]. That package is sized for European-style cadastral traverses where 100-gon right-angle offsets and 50-gon intermediate points are common field-book entries.
Limitations, Failure Modes, and Sourcing Notes
The horizontal circle on an auto level is the weakest angle-reading element in the surveyor's kit, with typical resolution no better than 1° (360° models) or 0.5 gon (400-gon models), so a parallax error between the index mark and the printed circle can easily eat the entire readability budget; the standard mitigation is to view the circle with the eyepiece roughly perpendicular to avoid the offset [S1][S5]. Compensator range on the units reviewed is ±15' (about ±0.25°), so any foot-screw mis-leveling beyond that disables auto-leveling and the operator is back to a manual instrument; the SitePro 24X/28X/32X list ±15' compensator range with ±0.3" setting accuracy, while the ALTAS 400-gon version lists ±0.6" setting accuracy, the looser figure reflecting its broader 1 km accuracy step at 2.5 mm at 20X [S1][S5].
Ingress rating IP54 is the published baseline across both 360° (SitePro 32X package) and 400-gon (ALTAS) bodies, meaning dust-protected and splash-resistant but not jet-washable, and the working-temperature window of -20°C to +50°C on the ALTAS defines the practical operating envelope for both graduation options on the same chassis [S5]. For related reading on how an automatic level fits between a laser level and a theodolite on the angle-accuracy ladder, the automatic level cost drivers breakdown tracks compensator and optics rather than circle graduation.
Track these signals before specifying: confirm whether the OEM ships the same SKU in both 360° and 400-gon dials (South does, SitePro does not list 400 gon), verify that 1 km double-run leveling accuracy is published for the magnification step you need, and require IP54 plus a published operating-temperature range on the datasheet.