GNSS archaeological site mapping: contexts, control and provenance

Define the archaeological mapping objects

An archaeological survey can include trenches, context boundaries, finds, surface features and control points. Agree what each coordinate represents with the project’s archaeological recording system. A find spot, a context centroid and the edge of an excavation are different spatial objects. Keep their definitions clear so a precise position does not become detached from the interpretation it was meant to support.

Use the project’s stable context and find identifiers, with separate observation identifiers where needed. Avoid creating a parallel survey numbering system that cannot be reconciled with the excavation record. The mapping plan should describe required attributes, photographs and review responsibilities. Spatial precision is useful only when it is connected to the correct archaeological record and provenance.

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Establish a documented site reference

Determine whether the site uses an external coordinate reference, a local excavation grid or both. Record the relationship between them, including origin, orientation, scale and height reference. A grid drawn on a site plan may be convenient for excavation but insufficient for exchange with a regional GIS unless its external connection is documented.

Test supplied control before relying on it. Confirm mark identity, coordinate version and any transformations already applied. Keep original local coordinates when they are part of the established site record, and create transformed outputs through a documented operation. Assigning an external CRS label to local grid numbers does not place the excavation correctly in that external reference.

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Protect and describe control points

Place or adopt control with attention to excavation progress, site protection requirements and access. Distinguish durable site control from temporary working marks that may be removed during digging. Record descriptions and photographs that another authorised crew can use. A control point should be recoverable without disturbing the feature or context it supports.

Review mark status as trenches expand or nearby work changes the ground. If a mark is damaged or uncertain, suspend its use until checked. Preserve its earlier history rather than deleting it from the record. Clear control status helps prevent a later survey from mixing observations based on stable references with those derived from a disturbed temporary mark.

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Choose the measurement method for each feature

Open site areas may be suitable for GNSS, while trenches, walls, trees and nearby buildings can obstruct satellite observations. Assess each feature location and the required result. A fixed RTK status is useful evidence but should be considered with the environment, repeated observations and independent checks. It does not replace a defined observation point or a correct antenna setup.

Use total station observations or another suitable method where GNSS cannot support the task. Keep the connection between methods and control explicit. If an offset is used, record its geometry and method. Do not move the antenna to a convenient location and assign that coordinate to a find or context without preserving the relationship between the measured and interpreted positions.

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Record boundaries as observations and interpretations

Context boundaries may be irregular, incomplete or partly interpreted. Agree which segments are directly visible and which are inferred. Capture enough vertices to represent meaningful shape changes without implying detail that was not observed. Keep interpretation status in the attributes or recording system. A clean polygon outline should not conceal uncertainty in the archaeological evidence.

Record vertical observations with their physical meaning, such as a context surface, base or specific feature point. Distinguish these from a generic terrain elevation. Where a feature is recorded at several excavation stages, preserve the stage and date. This prevents observations from different physical states being combined into a single apparently simultaneous three-dimensional surface.

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Link survey evidence to the site archive

Tie photographs, sketches and notes to context, find and observation identifiers. Include enough context in photographs to locate the observed feature within the trench or site. Keep the original measurement file and any coordinate transformations with the record. An exported drawing alone may lose the relationship between a geometry and the field evidence used to create it.

Test identifiers and attributes through the intended archive format. Leading zeros, long codes and special characters should survive export and import. Review a sample jointly with the archaeological team before collecting the entire site. This can reveal mismatches between survey terminology and recording practice while they are still inexpensive to correct.

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Check spatial consistency without rewriting interpretation

Observe control checks and repeat selected points according to the survey plan. Inspect linework for incorrect connections, duplicate identifiers and unexpected coordinate changes. Compare the map with field sketches and photographs. Positional review and archaeological interpretation are related but distinct responsibilities, and a geometric edit should not silently change the interpretation of a context.

When a discrepancy is found, preserve the original observation and document the correction or review decision. Do not shift a feature merely to align it with a previous plan unless the evidence supports that operation. Different survey dates may describe real changes in excavation extent or recording stage. The review must distinguish those changes from reference or transcription problems.

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Deliver a reproducible spatial archive

Provide the coordinate and height references, control descriptions, observation methods, dates and transformation history. Include a data dictionary explaining geometry types, identifiers and interpretation status. State which positions are measured directly, derived from offsets or digitised from another source. These distinctions allow future researchers to assess the spatial evidence appropriately.

Preserve local and external coordinate versions where both are part of the project record. Keep release versions and unresolved issues with the archive rather than only in personal notes. A dependable GNSS contribution to archaeology is more than a set of accurate-looking coordinates. It is a traceable connection between physical observations, site recording practice and the reference framework that lets later work interpret them.

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Preserve local excavation coordinates during transformation

An excavation may have years of records tied to a local grid before a new GNSS connection is established. Keep those original grid values as part of the archive. Derive external coordinates through a documented transformation using suitable control, and record the operation version. Replacing the old numbers in place can make earlier plans and notebooks difficult to interpret even when the new external positions are useful.

Test the transformation on independent points not used to determine it. Examine differences across the site rather than only near the grid origin. If older records have uneven quality, describe that limitation instead of stretching the transformation until every feature appears to match. The archive should let future researchers distinguish an original measurement, a transformed coordinate and a later interpretation of the relationship between historical site plans.

Include a small conversion table in the archive with original local values, derived external values, point identifiers and the transformation release. This provides a practical test for future software migrations. The table should use accepted control and independent checks clearly labelled by role. It does not replace the complete parameter definition, but it helps another researcher verify that the archived operation is being applied in the intended direction and with the correct units. Keep the table with both the site plan and the transformation documentation.

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