{
	"symptoms": [
		{
			"symptomId": "osgb36_etrs89_shift",
			"description": "Coordinates shifted by 1-5 meters",
			"keywords": ["OSGB36", "ETRS89", "shifted", "BNG", "meters"],
			"causes": [
				{
					"likelihood": "high",
					"cause": "OSGB36 and ETRS89 datum difference not accounted for",
					"description": "Direct comparison of OSGB36 and ETRS89 coordinates without transformation",
					"indicators": [
						"Shift varies across the UK (larger at edges)",
						"Systematic offset in one direction",
						"Data from different sources"
					]
				},
				{
					"likelihood": "medium",
					"cause": "Using Helmert transformation instead of OSTN15",
					"description": "7-parameter Helmert transformation is only accurate to ~5m",
					"indicators": ["Residual errors after transformation", "Errors vary spatially"]
				}
			],
			"solutions": [
				{
					"forCause": "OSGB36 and ETRS89 datum difference not accounted for",
					"steps": [
						"Identify the datum of each dataset (OSGB36 vs ETRS89)",
						"Use OSTN15 transformation for conversion",
						"Apply transformation using official OS tools or PROJ"
					],
					"prevention": "Always document datum and use OSTN15 for transformations"
				},
				{
					"forCause": "Using Helmert transformation instead of OSTN15",
					"steps": [
						"Identify that Helmert was used",
						"Re-transform using OSTN15 grid",
						"Validate against known control points"
					],
					"prevention": "Always use OSTN15 for OSGB36/ETRS89 transformations in GB"
				}
			]
		},
		{
			"symptomId": "bng_ni_mismatch",
			"description": "Northern Ireland data doesn't align with GB data",
			"keywords": ["Northern Ireland", "BNG", "ITM", "Ireland", "doesn't align"],
			"causes": [
				{
					"likelihood": "high",
					"cause": "Using BNG for Northern Ireland data",
					"description": "British National Grid does not cover Northern Ireland",
					"indicators": ["Data is for Northern Ireland locations", "Attempting to use EPSG:27700"]
				}
			],
			"solutions": [
				{
					"forCause": "Using BNG for Northern Ireland data",
					"steps": [
						"Use Irish Transverse Mercator (EPSG:2157) for Northern Ireland",
						"Transform both datasets to ETRS89 for cross-border work",
						"Reprojects if needed for UK-wide analysis"
					],
					"prevention": "Use ITM for Northern Ireland, BNG for GB only"
				}
			]
		},
		{
			"symptomId": "grid_reference_error",
			"description": "BNG grid reference not recognized or incorrect",
			"keywords": ["grid reference", "OS grid", "TQ", "invalid", "format"],
			"causes": [
				{
					"likelihood": "high",
					"cause": "Invalid grid reference format",
					"description": "BNG grid references must follow specific format (e.g., TQ 123 456)",
					"indicators": ["Error parsing grid reference", "Unexpected coordinate values"]
				},
				{
					"likelihood": "medium",
					"cause": "Grid reference precision mismatch",
					"description": "Grid references can be 6, 8, or 10 figures with different precision",
					"indicators": ["Coordinates seem rounded", "Location is approximate"]
				}
			],
			"solutions": [
				{
					"forCause": "Invalid grid reference format",
					"steps": [
						"Verify grid reference follows format: 2 letters + even number of digits",
						"Check letter pair is valid for UK (e.g., TQ, SO, NY)",
						"Ensure correct number of digits (6, 8, or 10)"
					],
					"prevention": "Validate grid references before processing"
				},
				{
					"forCause": "Grid reference precision mismatch",
					"steps": [
						"Determine required precision",
						"Use 10-figure grid references for maximum precision (1m)",
						"Document precision level in metadata"
					],
					"prevention": "Standardize grid reference precision for project"
				}
			]
		},
		{
			"symptomId": "wgs84_etrs89_drift",
			"description": "Small but systematic offset between GPS data and UK survey data",
			"keywords": ["GPS", "WGS84", "ETRS89", "drift", "small offset", "cm"],
			"causes": [
				{
					"likelihood": "high",
					"cause": "ETRS89/WGS84 epoch difference",
					"description": "ETRS89 is fixed to European plate while WGS84 follows global ITRF",
					"indicators": [
						"Offset is small (few cm to dm)",
						"Consistent across area",
						"Increasing over time"
					]
				}
			],
			"solutions": [
				{
					"forCause": "ETRS89/WGS84 epoch difference",
					"steps": [
						"For most applications (>1m accuracy), treat as equivalent",
						"For high-precision work, apply epoch transformation",
						"Document which realization is being used"
					],
					"prevention": "Specify datum realization in project documentation"
				}
			]
		}
	]
}