Occupancy grid map concept in ROS - Mini Project: Build & Apply

Occupancy Grid Map Concept in ROS

📖 Scenario: You are building a simple robot mapping system using ROS. The robot needs to keep track of which areas in its environment are free and which are occupied. This is done using an occupancy grid map, which is like a grid of squares where each square can be free, occupied, or unknown.

🎯 Goal: Create a basic occupancy grid map in ROS by setting up the map data, configuring the map size, filling the map with occupancy values, and publishing the map so other parts of the robot system can use it.

📋 What You'll Learn

Create a ROS node that initializes an occupancy grid map

Set the map size and resolution using a configuration variable

Fill the occupancy grid data with values representing free and occupied cells

Publish the occupancy grid map on a ROS topic

💡 Why This Matters

🌍 Real World

Robots use occupancy grid maps to understand their surroundings and avoid obstacles while navigating.

💼 Career

Understanding occupancy grid maps is essential for robotics engineers working on autonomous navigation and mapping.

1

Create the OccupancyGrid message with header and info

Create a variable called map_msg as a nav_msgs.msg.OccupancyGrid message. Set its header.frame_id to "map" and initialize info as a nav_msgs.msg.MapMetaData object.

ROS

# Your code here to create map_msg and set header.frame_id and info

Hint

Use OccupancyGrid() to create the map message and set the frame ID to "map".

2

Set map size and resolution configuration

Create a variable called resolution and set it to 0.1. Set map_msg.info.width to 100 and map_msg.info.height to 100. Also set map_msg.info.resolution to resolution.

ROS

import rospy
from nav_msgs.msg import OccupancyGrid, MapMetaData

map_msg = OccupancyGrid()
map_msg.header.frame_id = "map"
map_msg.info = MapMetaData()

# Set resolution and map size
# Your code here

Hint

Set the map width and height to 100 cells each and resolution to 0.1 meters per cell.

3

Fill the occupancy grid data with free and occupied cells

Create a list called data with map_msg.info.width * map_msg.info.height elements. Set the first 5000 elements to 0 (free space) and the rest to 100 (occupied space). Assign this list to map_msg.data.

ROS

import rospy
from nav_msgs.msg import OccupancyGrid, MapMetaData

map_msg = OccupancyGrid()
map_msg.header.frame_id = "map"
map_msg.info = MapMetaData()

resolution = 0.1
map_msg.info.width = 100
map_msg.info.height = 100
map_msg.info.resolution = resolution

# Fill map_msg.data with occupancy values
# Your code here

Hint

Create a list with 5000 zeros followed by occupied cells with value 100.

4

Publish the occupancy grid map on a ROS topic

Create a ROS publisher called map_pub that publishes OccupancyGrid messages on the topic "/map". Publish map_msg once using map_pub.publish(map_msg).

ROS

import rospy
from nav_msgs.msg import OccupancyGrid, MapMetaData

map_msg = OccupancyGrid()
map_msg.header.frame_id = "map"
map_msg.info = MapMetaData()

resolution = 0.1
map_msg.info.width = 100
map_msg.info.height = 100
map_msg.info.resolution = resolution

data = [0]*5000 + [100]*(map_msg.info.width * map_msg.info.height - 5000)
map_msg.data = data

# Create publisher and publish map_msg
# Your code here

Hint

Initialize the ROS node and create a publisher for the '/map' topic.

Practice

1.

What does an occupancy grid map represent in ROS?

easy

A. A grid showing which areas are free, occupied, or unknown

B. A list of robot commands to move in space

C. A 3D model of the robot's shape

D. A sensor data stream from the robot's camera

Solution

1. Step 1: Understand the purpose of occupancy grid maps

   Occupancy grid maps divide space into small cells to mark if they are free, occupied, or unknown.

2. Step 2: Match the description to the options

   A grid showing which areas are free, occupied, or unknown correctly describes this as a grid showing cell states. Other options describe unrelated concepts.

3. Final Answer:

   A grid showing which areas are free, occupied, or unknown -> Option A

4. Quick Check:

   Occupancy grid = space divided into free/occupied/unknown cells [OK]

Hint: Occupancy grid = map of free or blocked spaces [OK]

Common Mistakes:

• Confusing occupancy grid with robot commands
• Thinking it is a 3D robot model
• Mixing it up with raw sensor data

2.

Which data type correctly represents the occupancy grid map data in ROS?

nav_msgs::OccupancyGrid map;
// What type is map.data?

easy

A. bool[][]

B. std::string

C. std::vector<int8_t>

D. float[]

Solution

1. Step 1: Recall the ROS message type for occupancy grid data

   The map.data field is a list of signed 8-bit integers representing cell states.

2. Step 2: Match the data type to the options

   std::vector<int8_t> matches the expected type. Other options do not fit the ROS message definition.

3. Final Answer:

   std::vector<int8_t> -> Option C

4. Quick Check:

   map.data type = std::vector<int8_t> [OK]

Hint: Occupancy data uses vector of int8_t in ROS [OK]

Common Mistakes:

• Assuming map.data is a string or float array
• Thinking it is a 2D bool array
• Confusing with sensor_msgs data types

3.

Given this snippet in ROS Python:

grid = [0, 100, -1, 0]
free_cells = sum(1 for cell in grid if cell == 0)
occupied_cells = sum(1 for cell in grid if cell == 100)
unknown_cells = sum(1 for cell in grid if cell == -1)
print(f"Free: {free_cells}, Occupied: {occupied_cells}, Unknown: {unknown_cells}")

What will be printed?

medium

A. Free: 1, Occupied: 1, Unknown: 2

B. Free: 1, Occupied: 2, Unknown: 1

C. Free: 2, Occupied: 0, Unknown: 2

D. Free: 2, Occupied: 1, Unknown: 1

Solution

1. Step 1: Count free cells (value 0) in the list

   grid has two zeros at positions 0 and 3, so free_cells = 2.

2. Step 2: Count occupied cells (value 100) and unknown cells (value -1)

   One 100 at position 1 (occupied_cells = 1), one -1 at position 2 (unknown_cells = 1).

3. Final Answer:

   Free: 2, Occupied: 1, Unknown: 1 -> Option D

4. Quick Check:

   Count values 0, 100, -1 correctly [OK]

Hint: Count each cell value carefully to avoid off-by-one errors [OK]

Common Mistakes:

• Miscounting zeros or other values
• Confusing unknown (-1) with free (0)
• Mixing occupied (100) with unknown (-1)

4.

Identify the error in this ROS Python code snippet that tries to update an occupancy grid cell:

map_data = [0, 0, 0, 0]
index = 2
map_data[index] = 100
print(map_data)

Why might this code fail in a real ROS occupancy grid update?

medium

A. map_data should be updated via the OccupancyGrid message, not a plain list

B. map_data is a tuple and cannot be changed

C. Index 2 is out of range for map_data

D. 100 is not a valid occupancy value

Solution

1. Step 1: Check data type and mutability

   map_data is a list here, so assignment works in Python, no error from mutability.

2. Step 2: Understand ROS occupancy grid update rules

   In ROS, occupancy grid data must be updated through the OccupancyGrid message interface, not by directly modifying a local list copy.

3. Final Answer:

   map_data should be updated via the OccupancyGrid message, not a plain list -> Option A

4. Quick Check:

   Update occupancy grid via ROS message, not local list [OK]

Hint: Update occupancy grid through ROS message, not local list [OK]

Common Mistakes:

• Assuming direct list change updates ROS map
• Thinking index 2 is invalid
• Believing 100 is invalid occupancy value

5.

You want to create a 5x5 occupancy grid map in ROS where the center cell is occupied (value 100), edges are unknown (-1), and others are free (0). Which Python code correctly creates the data list for this map?

# Options:
# A
# data = [100 if i == 12 else -1 if i % 5 == 0 or i % 5 == 4 else 0 for i in range(25)]

# B
# data = [0]*25
# data[12] = 100
# for i in range(25):
#     if i % 5 == 0 or i % 5 == 4 or i < 5 or i > 19:
#         data[i] = -1

# C
# data = [-1]*25
# data[12] = 100
# for i in range(25):
#     if i % 5 != 0 and i % 5 != 4 and 5 <= i <= 19:
#         data[i] = 0

# D
# data = [0 if i != 7 else 100 for i in range(25)]
# for i in range(25):
#     if i % 5 == 0 or i % 5 == 4 or i < 5 or i > 19:
#         data[i] = -1

hard

A. Creates data with center 100, edges -1, others 0 in one list comprehension

B. Starts with all 0s, sets center 100, then sets edges to -1 in a loop

C. Starts with all -1s, sets center 100, then sets inner cells to 0 in a loop

D. Starts with zeros except center 100, then sets edges -1 in a loop

Solution

1. Step 1: Understand the map layout requirements

   Center cell (index 12) = 100, edges = -1, others = 0 in a 5x5 grid (indexes 0-24).

2. Step 2: Analyze each option's logic

   Starts with all 0s, sets center to 100, then sets edges to -1 in a loop, matching requirements exactly.

3. Final Answer:

   Starts with all 0s, sets center 100, then sets edges to -1 in a loop -> Option B

4. Quick Check:

   Initialize edges as -1, center 100, inner cells 0 [OK]

Hint: Initialize edges first, then set center and free cells [OK]

Common Mistakes:

• Confusing edge indexes with inner cells
• Overwriting center cell after setting edges
• Using incorrect conditions for edges