# Working with ArUco Markers
**URL:** https://forum.hello-robot.com/t/working-with-aruco-markers/243
**Category:** Knowledge Base
**Created:** [July 8, 2021, 6:31pm UTC](https://forum.hello-robot.com/t/working-with-aruco-markers/243 "2021-07-08T18:31:25Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@kpputhuveetil](https://forum.hello-robot.com/u/kpputhuveetil)
#### Post date: [July 8, 2021, 6:31pm UTC](https://forum.hello-robot.com/t/working-with-aruco-markers/243/1 "2021-07-08T18:31:25Z")
Hello!
I’ve been working with the detect\_aruco\_markers node from stretch\_core in the stretch\_ros repository and have been noticing some discrepancies between the detected position of a given set of Aruco tags and their actual position. See the attached image for an example of what this looks like.
I created this image by simply using Rviz to display the MarkerArrays published by detect\_aruco\_markers to the /aruco/marker\_array and /aruco/axes topics, as well as the PointCloud output by the depth camera. I added some annotations to hopefully make the issue a little more clear.
The tags circled in green are those where the generated marker array and the tag are aligned (it’s a little hard to tell but the detected axes are visible). All of those tags are stickers that came with the robot, either on its body or in the bag of extras.
The tags boxed in blue (came in the box of extras with the robot) or red (printed on paper) are those where there is poor alignment between the detected and actual position of the tag.
I’m having trouble identifying what may be causing the issue since there doesn’t seem to be a unifying characteristic between the tags that are affected and those that are not. Any insight you may have would be much appreciated. Thank you!
---
### Author:  [@hello-ck](https://forum.hello-robot.com/u/hello-ck)
#### Post date: [July 8, 2021, 9:11pm UTC](https://forum.hello-robot.com/t/working-with-aruco-markers/243/2 "2021-07-08T21:11:41Z")
Hello @kpputhuveetil!
This is an excellent question. It’s great that you’re working with ArUco markers that aren’t on the robot’s body. I don’t think we’ve provided clear documentation for this, so I’m going to take this opportunity to both answer your question and provide additional context.
**The ArUco Marker Dictionary**
The most likely cause of your issue is [`stretch_marker_dict.yaml`](https://github.com/hello-robot/stretch_ros/blob/master/stretch_core/config/stretch_marker_dict.yaml), which uses [YAML](https://en.wikipedia.org/wiki/YAML) to define a dictionary that holds information about ArUco markers.
If [detect\_aruco\_markers](https://github.com/hello-robot/stretch_ros/blob/master/stretch_core/nodes/detect_aruco_markers) doesn’t find an entry in [`stretch_marker_dict.yaml`](https://github.com/hello-robot/stretch_ros/blob/master/stretch_core/config/stretch_marker_dict.yaml) for a particular ArUco marker ID number, it uses the [`default`](https://github.com/hello-robot/stretch_ros/blob/92d077558c8e6553294f498c8a1e8913f5a4864d/stretch_core/config/stretch_marker_dict.yaml#L70) entry. For example, most robots have shipped with the following `default` entry
```auto
'default':
'length_mm': 24
'use_rgb_only': False
'name': 'unknown'
'link': None
```
and the following entry for the ArUco marker on the top of the wrist
```auto
'133':
'length_mm': 23.5
'use_rgb_only': False
'name': 'wrist_top'
'link': 'link_aruco_top_wrist'
```
.
It’s good practice to add an entry to [`stretch_marker_dict.yaml`](https://github.com/hello-robot/stretch_ros/blob/master/stretch_core/config/stretch_marker_dict.yaml) for each ArUco marker you use.
- The dictionary key for each entry is the ArUco marker’s ID number or `default`. For example, the entry shown above for the ArUco marker on the top of the wrist assumes that the marker’s ID number is `133`.
- A mismatch between `length_mm` and your ArUco markers is probably the source of your pose estimation errors. The next section provides more details.
- If `use_rgb_only` is `True`, [detect\_aruco\_markers](https://github.com/hello-robot/stretch_ros/blob/master/stretch_core/nodes/detect_aruco_markers) will ignore depth images from the [Intel RealSense D435i depth camera](https://www.intelrealsense.com/depth-camera-d435i/) when estimating the pose of the marker and will instead only use RGB images from the D435i.
- `name` is used for the `text` string of the ArUco marker’s [ROS Marker](http://docs.ros.org/en/melodic/api/visualization_msgs/html/msg/Marker.html) in the [ROS MarkerArray Message](http://docs.ros.org/en/melodic/api/visualization_msgs/html/msg/MarkerArray.html) published by the [detect\_aruco\_markers](https://github.com/hello-robot/stretch_ros/blob/master/stretch_core/nodes/detect_aruco_markers) ROS node.
- `link` is currently used by [stretch\_calibration](https://github.com/hello-robot/stretch_ros/blob/master/stretch_calibration/nodes/collect_head_calibration_data). It is the name of the link associated with a body-mounted ArUco marker in [the robot’s URDF](https://github.com/hello-robot/stretch_ros/blob/master/stretch_description/urdf/stretch_aruco.xacro).
**Carefully Measure Custom ArUco Markers**
The `length_mm` value used by [detect\_aruco\_markers](https://github.com/hello-robot/stretch_ros/blob/master/stretch_core/nodes/detect_aruco_markers) is important for estimating the pose of an ArUco marker. If the actual width and height of the marker do not match this value, then pose estimation will be poor.
In practice, you should measure any marker by hand. For example, at [Hello Robot](https://hello-robot.com) we’ve printed out `180mm` markers in the past only to later learn that they’re actually `179mm` markers. Similar discrepancies with early wrist markers resulted in poor calibration.
**Official Guide to Creating New ArUco Markers**
At Hello Robot, we’ve used the following guide when generating new ArUco markers.
We generate ArUco markers using a 6x6 bit grid (36 bits) with 250 unique codes. This corresponds with [DICT\_6X6\_250 defined in OpenCV](https://docs.opencv.org/3.4/d9/d6a/group__aruco.html). We generate markers using [this online ArUco marker generator](http://chev.me/arucogen/) by setting the `Dictionary` entry to `6x6` and then setting the `Marker ID` and `Marker size, mm` as appropriate for the specific application. As described above, we measure the actual marker we use by hand prior to adding an entry for it to [`stretch_marker_dict.yaml`](https://github.com/hello-robot/stretch_ros/blob/master/stretch_core/config/stretch_marker_dict.yaml).
We select marker ID numbers using the following ranges.
- 0 - 99 : reserved for users
- 100 - 249 : reserved for official use by Hello Robot Inc.
- 100 - 199 : reserved for robots with distinct sets of body-mounted markers
- Allows different robots near each other to use distinct sets of body-mounted markers to avoid confusion. This could be valuable for various uses of body-mounted markers, including calibration, visual servoing, visual motion capture, and multi-robot tasks.
- 5 markers per robot = 2 on the mobile base + 2 on the wrist + 1 on the shoulder
- 20 distinct sets = 100 available ID numbers / 5 ID numbers per robot
- 200 - 249 : reserved for official accessories
- 245 for the prototype docking station
- 246-249 for large floor markers
When coming up with this guide, we expected the following:
- Body-mounted accessories with the same ID numbers mounted to different robots could be disambiguated using the expected range of 3D locations of the ArUco markers on the calibrated body.
- Accessories in the environment with the same ID numbers could be disambiguated using a map or nearby observable features of the environment.
I hope you find this helpful!
Best wishes,
Charlie
_Charlie Kemp, PhD_
_co-founder & CTO_
_Hello Robot Inc._
_[http://charliekemp.com](http://charliekemp.com/)_
---
### Author:  [@kpputhuveetil](https://forum.hello-robot.com/u/kpputhuveetil)
#### Post date: [July 8, 2021, 9:26pm UTC](https://forum.hello-robot.com/t/working-with-aruco-markers/243/3 "2021-07-08T21:26:42Z")
Thank you so much Dr. Kemp! This is extremely helpful!
You were right - stretch\_marker\_dict.yaml was indeed the cause of the problem. When I asked the question, I actually had been editing the yaml file to include the additional tags. However, I guess I wasn’t careful enough in ensuring that the tabs/spaces in the file were correct and as a result, some of the new entries were ignored and the default settings were applied. Fixing the formatting in the file has resolved the problem.
Thank you again for your detailed reply. I’ll definitely be keeping this additional context in mind for further work with the ArUco markers.