DGPS (Differential GPS) — spatial correlation of bias errors DGPS relies on the assumption that most positioning biases are nearly the same within a local area. Up to what approximate baseline length does this assumption hold well for routine surveying?

Introduction / Context:
Differential GPS improves positional accuracy by applying corrections from a reference station to a rover. The effectiveness depends on how similar the error sources (ionospheric/tropospheric delays, satellite ephemeris, clock errors) are over the separation between stations, known as the baseline length.

Given Data / Assumptions:

• Standard single-frequency (or dual-frequency) code/phase DGPS for typical engineering surveys.
• Moderate solar activity and mid-latitude conditions.
• Desire for decimetre to sub-metre results with real-time corrections.

Concept / Approach:

Error fields decorrelate with distance. Within roughly 100 km, atmospheric and orbital components remain sufficiently correlated that reference corrections significantly reduce rover errors. Beyond that, residuals grow and additional modeling or network RTK approaches are preferred.

Step-by-Step Solution:

Verification / Alternative check:

Network RTK and SBAS solutions extend ranges by modeling spatial gradients; classic single-base DGPS commonly cites ~100 km as an upper practical limit for good performance.

Why Other Options Are Wrong:

• 25–75 km are usable but overly restrictive as a hard limit.
• 1250 km far exceeds the range over which biases remain similar.

Common Pitfalls:

• Ignoring space-weather/ionospheric storms which can shrink effective ranges.
• Assuming uniform performance for single-frequency receivers across seasons and latitudes.

Final Answer:

100 km.