# cartographer/sensor/fixed_frame_pose_data.h

## Overview

`FixedFramePoseData` represents an absolute pose measurement in a fixed frame — for example GPS position, or an external pose estimate. Unlike relative sensor data (IMU, odometry), it is used directly in the global optimization: the pose graph adds residuals that pull each node's global pose toward the fixed-frame measurement, which helps bind the map to the world frame.

## Struct: `cartographer::sensor::FixedFramePoseData`

```cpp
struct FixedFramePoseData {
  common::Time time;
  absl::optional<transform::Rigid3d> pose;
};
```

- `time` — Timestamp of the measurement.
- `pose` — Optional pose in the fixed frame. The `absl::optional` allows a measurement with no usable pose (e.g. a GPS fix with no lock) to be delivered without discarding the timestamp.

## Serialization

```cpp
proto::FixedFramePoseData ToProto(const FixedFramePoseData& pose_data);
FixedFramePoseData FromProto(const proto::FixedFramePoseData& proto);
```

`proto::FixedFramePoseData` stores the timestamp and an optional `transform.proto.Rigid3d`:

```protobuf
message FixedFramePoseData {
  int64 timestamp = 1;
  transform.proto.Rigid3d pose = 2;
}
```

`ToProto` omits `pose` when it is not set; `FromProto` reconstructs the `absl::optional` from `has_pose()`.

## Usage

```cpp
sensor::FixedFramePoseData pose_data{time, pose};  // or {time, absl::nullopt}
trajectory_builder->AddFixedFramePoseData(sensor_id, pose_data);
```

## Role in the SLAM Pipeline

- **Trajectory builder interface** — `AddFixedFramePoseData` accepts these measurements; they are stored per trajectory in the optimization problem.
- **Optimization** — During `Solve`, each trajectory with fixed-frame pose data has a `fixed_frame_origin_in_map` transform computed and residual blocks (in `OptimizationProblem2D`/`OptimizationProblem3D`) pull the trajectory's nodes toward the measured poses, constraining the map to the fixed frame.
- **Serialization** — per-trajectory fixed-frame pose data is stored in the serialization format via `proto::FixedFramePoseData` in `serialization.proto`.

## See Also

- [Data](data.md) — Base class and `Dispatchable` wrapping
- [OdometryData](odometry_data.md) — Relative pose measurement
- [OptimizationProblemInterface](../mapping/internal/optimization/optimization_problem_interface.md) — Consumes fixed-frame pose data