# Geocache Letterbox Munzee Stage-Cache Puzzle-Hunt Coordinate Validator

Validate GC-codes (base-31 with offset, or hex era), place caches on the 81-cell D/T Fizzy grid, chain multi-stage bearing projections into WGS84 waypoints, and cross-check puzzle finals against checker coordinates.

> Canonical page: https://elysiatools.com/en/tools/geocache-letterbox-munzee-stage-cache-puzzle-hunt-coordinate-validator

- **Category:** Games

- **Keywords:** gc code decoder, geocaching puzzle solver, waypoint projection, fizzy grid, bearing distance calculator, letterbox clue, munzee pin, multi-cache stages, wgs84 decimal dms, coordinate checker

## Overview

An offline validator for geocaching-style puzzle hunts.

GC-code engine: the 31-character alphabet 0-9ABCDEFGHJKMNPQRTVWXYZ (I, L, O, S, U excluded; typed S and O normalize to 5 and 0). Ids 0–65535 were issued as plain hexadecimal (GC0…GCFFFF); from id 65536 (GCG000) the code is base-31 with a +411120 offset — id = base31(suffix) − 411120. Verified against published vectors (GC2SOSO = GC25050 = 1585032) and the era boundary.

Projection engine: great-circle forward solution on a sphere of radius 6,371,008.8 m — φ₂ = asin(sin φ₁ cos δ + cos φ₁ sin δ cos θ), λ₂ = λ₁ + atan2(sin θ sin δ cos φ₁, cos δ − sin φ₁ sin φ₂). Legs are chained so a multi-cache stage tour resolves in one pass; every waypoint is shown in both WGS84 decimal and the N 47° 36.600′ listing format.

Checker: haversine distance plus bearing from your computed final to the checker coordinates, with a tolerance verdict and a rule-of-thumb for how far a 1° bearing error drifts.

D/T: the 81-cell Fizzy grid (D and T each 1.0–5.0 in 0.5 steps), with your cell highlighted and #81 (D5/T5) marked.

Type guidance covers multi-cache stage conventions, mystery checkers, letterbox-hybrid stamps and clue offsets (Atlas Quest lineage), EarthCache, Wherigo cartridges and Munzee QR pins (no D/T scale). Structural validation only — no live geocaching.com lookup.

## Inputs

- **GC Code** (text): e.g. GC2SOSO (S reads as 5)
- **Cache / Pin Type** (select)
- **Difficulty D (1–5, half steps)** (number): 2.5
- **Terrain T (1–5, half steps)** (number): 2
- **Posted / Stage-1 Latitude (WGS84 decimal)** (number): 47.6100
- **Posted / Stage-1 Longitude (WGS84 decimal)** (number): 8.5500
- **Projection Legs (one per line: bearing°, meters)** (textarea): 42.5, 315
128, 210
- **Checker / Final Latitude (optional)** (number): 47.6109
- **Checker / Final Longitude (optional)** (number): 8.5550
- **Checker Tolerance (meters)** (number): 10

## When to use

- Decoding GC-codes across both the legacy hexadecimal era and modern base-31 offset formats to inspect numeric IDs.
- Solving multi-stage mystery caches or letterbox clues that require chaining multiple bearing and distance projections from an initial stage.
- Cross-referencing computed puzzle hunt coordinates against coordinate checker values using configurable meter tolerance thresholds.

## How it works

- Enter a GC-code along with the cache type, difficulty, and terrain ratings to decode the numeric ID and highlight the corresponding cell on the 81-cell Fizzy grid.
- Input the stage-1 or posted WGS84 decimal coordinates and add one or more projection legs defined as bearing in degrees and distance in meters.
- Optionally enter checker coordinates and an error tolerance in meters to calculate haversine distance and evaluate whether the projected final falls within the accepted range.

## Use cases

- Calculating multi-stage field offsets and final coordinates for multi-caches and letterbox hybrids.
- Verifying home puzzle solutions against geochecker solutions before heading out into the field.
- Tracking milestone coverage on the 81-combination Difficulty/Terrain Fizzy grid for challenge caches.

## Frequently asked questions

### How does the tool handle GC-code base-31 and hexadecimal eras?

Codes up to GCFFFF are decoded as standard hexadecimal IDs (0 to 65535), while codes starting from GCG000 use the 31-character geocaching alphabet with a +411120 offset.

### What happens if I type confusing characters like S or O into the GC-code field?

The validator automatically normalizes ambiguous characters like S to 5 and O to 0 to match canonical GC-code standards.

### How are multiple projection legs calculated?

Legs are chained sequentially using forward great-circle equations on a 6,371,008.8-meter spherical model, feeding the result of each step into the next.

### In what coordinate formats are the calculated waypoints displayed?

Results are output in standard WGS84 decimal degrees alongside the traditional DD° MM.mmm′ geocaching listing format.

### Does this validator connect to live listing services?

No, it functions entirely as an offline mathematical and structural validator without making external network queries.

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