# Occupancy Grid Mapping

In Robot Localization, it is assumed that the robot is given a map in advance. This is sometimes not the case.

Acquiring maps with mobile robots is a challenging task, because:

1. The hypothesis space of all possible maps is huge (infinite). Under discrete approximations like grid approximations, this space remains computationally intractable for many Bayesian approaches.
2. Learning maps is a chicken-and-egg problem, hence it is often referred to as the simultaneous localization and mapping (SLAM) problem.

## Factors in hardness of mapping

size
the larger the environment, the more difficult
noise in perception
noise-free sensors lead to simple mapping solutions
perceptual ambiguity
the more frequently different places look alike, the harder to establish correspondence in different points in time
cycles
cycles in the environment are particularly difficult to map. Cycles make robots return via different paths, resulting in large amounts of accumulated odometric errors.

## Occupancy Grid Mapping

We assume some oracle informs us of the exact robot path during mapping.

Occupancy grid maps address the problem of generating consistent maps from noisy and uncertain measurement data, under the assumption that the robot pose is known. This technique is not useful in generating maps for path-planning and navigating, since no robot odometry is perfect.

The gold standard of any occupancy grid mapping algorithm is to calculate the posterior over maps given the data:

\begin{equation} p( m | z_{1:t},x_{1:t}) \end{equation}

The controls $$u_{1:t}$$ play no role, since the robot pose is known. The most common of occupancy maps are 2-D floorplan maps: a 2-D slice of the 3D world. These techniques generalize to 3D at significant computational expense.

Let $$m_{i}$$ done the grid cell with index $$i$$. An occupancy grid map partitions the space into finitely many grid cells:

\begin{equation} m=\sum_{i} \mathbf{m}_{i} \end{equation}

Each $$m_i$$ is attached a binary occupancy value, which specifies whether the cell is occupied.

With this decomposition, the problem remains computationally intractable. The standard approach further breaks down the problem, instead estimating:

\begin{equation} p\left(\mathbf{m}_{i} | z_{1: t}, x_{1: t}\right) \end{equation}

Assuming conditional independence between grid cells, the posterior over maps is approximated as:

\begin{equation} p\left(m | z_{1: t}, x_{1: t}\right)=\prod_i p\left(\mathbf{m}_{i} | z_{1: t}, x_{1: t}\right) \end{equation}

Occupancy is often represented in its log-odds form:

\begin{equation} l_{t, i}=\log \frac{p\left(\mathbf{m}_{i} | z_{1: t}, x_{1: t}\right)}{1-p\left(\mathbf{m}_{i} | z_{1: t}, x_{1: t}\right)} \end{equation}

The algorithm is as follows:

\begin{algorithm} \caption{Occupancy Grid Mapping} \label{occupancy_grid_mapping} \begin{algorithmic} \Procedure{Occupancy Grid Mapping}{$\{l_{t-1, i}\}, x_t, z_t$} \ForAll{cells $\mathbf{m}_i$} \If {$\mathbf{m}_i$ in perceptual field of $z_t$} \State $l_{t,i} = l_{t-1,i} + \mathrm{inverse sensor model}(\mathbf{m}_i, x_t,z_t) - l_0$ \Else \State $l_{t,i} = l_{t-1,i}$ \EndIf \State \Return $l_{t,i}$ \EndProcedure \end{algorithmic} \end{algorithm}

Occupancy grid mapping requires an inverse sensor model. One can also utilize the space claimed by the robot itself during mapping, by returning a large negative number for all grid cells occupied by the robot when at $$x_t$$.

## Multi-sensor integration

Different sensors have different characteristics, and are sensitive to different kinds of obstacles. There are 2 basic approaches to fusing data from multiple sensors:

1. Execute the algorithm with different sensor modalities, replacing the inverse sensor model accordingly.
1. This causes the result of Bayes filtering to be ill-defined.
2. Build separate maps for different sensor types, and integrate them using the most conservative estimate.

2 is the more appropriate approach.

## Inverting the Measurement Model

The occupancy grid mapping algorithm requires a marginalized inverse measurement model: $$p\left(\mathbf{m}_{i} | x, z\right)$$. It is termed “inverse” because it reasons from effects to causes.

We can do so by using Bayes rule, assuming $$p(m|x) = p(m)$$:

\begin{aligned} p(m | x, z) &=\frac{p(z | x, m) p(m | x)}{p(z | x)} \ &=\eta p(z | x, m) p(m) \end{aligned}

However, the occupancy grid mapping algorithm approximates the posterior over maps by its marginals, so the inverse model for the ith grid cell is obtained via:

\begin{equation} p\left(\mathbf{m}_{i} | x, z\right)=\eta \int_{m: m(i)=\mathbf{m}_{i}} p(z | x, m) p(m) d m \end{equation}

Which is impossible to compute given the large space of all maps. It is often approximated via sampling. One simple way is to generate random triplets of pose, measurements and map occupancy values for any grid cell $$m_i$$. From which we can learn a simple network to predict occupancy values.

## Maintaining Dependencies in Grid Cells

The standard algorithm decomposes the grid, making the assumption that the occupancy value in each grid cell is conditionally independent from the others. This is as strong assumption.

However, the full map posterior is not computable, due to the large number of possible maps defined over a grid. However, despite this, it is still maximizable (MAP). This leads to maps that are more consistent with the data, but requires full availability of data. Another downside is that the MAP map does not capture the residual uncertainty in the map.