A WiFi heat map is a floor plan coloured to show how a wireless measurement changes across a space. Each map shows one thing, such as signal strength, signal-to-noise ratio or channel overlap, so reading a survey well means knowing which map answers which question and what the colours are being compared against.
Where heat maps come from
During an on-site survey, a surveyor walks planned routes with survey software, clicking their position on the floor plan as they go. The software records readings at each point and fills in the space between them. At ehZee we use AirMagnet Survey Pro. A predictive model produces maps that look the same, but they are calculated from assumptions rather than measured, so always check which kind you are looking at.
Because the software fills gaps between readings, the map is only as good as the walk. Areas the surveyor could not enter, such as locked rooms or high racking aisles, may show colour that was estimated rather than measured. A good report says where that is the case.
The colours depend on the scale
Most tools default to green for good, yellow for marginal and red or grey for poor, but the thresholds are set by whoever runs the survey. A map can be made to look reassuring simply by choosing a generous threshold. Before reading any map, find the legend and check what values the colours represent, and whether those values match the requirements for the space.
The main types of heat map
Signal strength (coverage)
Shows the strongest signal a device would hear at each point, in dBm. Values closer to zero are stronger: −60 dBm is stronger than −75 dBm. Networks that carry voice or video are commonly designed to about −67 dBm or better; see Why −67 dBm is the WiFi design target. A secondary coverage map, showing the second-strongest access point, matters for roaming because devices need somewhere to move to.
Signal-to-noise ratio (SNR)
Shows how far the signal stands above background noise, in decibels. A strong signal in a noisy area can still perform badly, so the SNR map often explains problems that the coverage map does not. RSSI, SNR and signal strength explains the relationship.
Channel overlap or co-channel interference
Shows how many access points on the same channel can be heard at each point. Where several overlap, they take turns to transmit, which reduces capacity even though coverage looks excellent. This map is often the key to a network that has strong signal everywhere and still feels slow.
Data rate and throughput
From an active survey, these maps show the connection speed a device negotiated and the throughput it actually achieved. They show the combined effect of signal, noise, interference and configuration.
Number of access points heard
Shows how many access points are audible above a set level. Too few means no backup for roaming; too many on the same band suggests excess transmit power or too many radios.
Band-specific maps
Separate maps for 2.4, 5 and 6 GHz. Signals at higher frequencies lose more strength through walls, so coverage at 5 and 6 GHz is usually smaller than at 2.4 GHz from the same access point.
Common mistakes when reading heat maps
- Reading only the coverage map. Green coverage everywhere does not mean the network works. Check SNR and channel overlap too.
- Ignoring the device. Survey adapters often hear better than phones, tablets and scanners. Good surveys account for the weakest important device.
- Mixing up floors. Access points on the floors above and below contribute signal and interference. Look at multi-storey maps together.
- Forgetting the time of day. A survey in an empty building does not show the effect of people, stock or neighbouring networks at busy times.
- Treating predictive maps as measured. They are a design tool, not evidence.
Which map answers which question
| If the question is | Start with | Then check |
|---|---|---|
| Is there signal everywhere people work? | Signal strength | Band-specific maps for the devices that matter |
| Will calls survive walking around? | Secondary coverage | Number of access points heard |
| Why is it slow where the signal is strong? | Channel overlap | SNR, then throughput from an active survey |
| Is something other than WiFi causing trouble? | SNR | Spectrum analysis in the affected area |
| Did the changes work? | The same maps as the first survey | Compared side by side with the original |
A worked example
An office tenant complains that the WiFi is slow in the open-plan area in the middle of the floor. The survey report opens with a coverage map that is green from wall to wall, and the first reaction is that the network is fine and the problem must be the internet connection.
The channel overlap map tells a different story. In the open area, a device can hear several access points on the same 5 GHz channel, including two on the floor above, because transmit power is set high and nothing blocks the signal across the open floor. Those access points must take turns, so each one gets a fraction of the airtime. The throughput map from the active survey confirms it: speeds drop exactly where overlap is highest, even though signal is strong.
The fix is not more access points. It is a revised channel plan, lower transmit power so each access point serves a smaller area, and a review of channel widths. A validation survey afterwards shows the overlap map with far less red, and the throughput map matching.
Questions to ask about any heat map
- Is this map measured or predicted?
- Which band, which floor, and which device or adapter took the readings?
- What value does each colour represent, and what is the target for this area?
- Which areas were not walked, and where is the colour estimated?
- When was the survey done, and was the space in normal use?
Ask for the targets. A heat map without stated requirements is a picture, not a finding. Every survey report should say what signal, SNR and overlap targets each area was measured against, and why.
Using heat maps to make decisions
Heat maps work best as before-and-after evidence. A survey of an existing network shows where problems are; a design shows how changes should fix them; a validation survey after the work confirms they did. Keep the maps with your building records. When tenants change, walls move or new devices arrive, a fresh survey compared with the old one shows exactly what changed.
When reviewing maps with a designer, ask practical questions: which areas fall short of the target, which map shows the cause, and what specific change is recommended for each one.
How ehZee can help
We produce heat maps from on-site surveys with AirMagnet Survey Pro and explain them in plain language, with the targets used for each area stated up front. Our WiFi surveys service turns the maps into a ranked list of fixes, and we can validate the network again once the changes are made.
Common questions
Why is my heat map green but the WiFi still slow?
Strong signal does not guarantee capacity. Check the channel overlap and SNR maps, the number of users per access point and the internet connection itself. See Troubleshooting slow WiFi for a step-by-step approach.
Can I make my own heat map with a free app?
Simple apps can give a rough picture of signal strength. They usually cannot measure SNR, channel overlap or active throughput accurately, and they reflect only the device you use.
How often should a building be resurveyed?
Whenever the layout, use or network changes significantly, and when users report new problems. There is no fixed interval.