--- title: Read and Understand the Carpet Plot description: A practical guide explaining how to interpret carpet plots in solar irradiance data analysis, including detection of time shifts, data gaps, nighttime anomalies, seasonal patterns, and early-stage data validation checks. --- [![EKO Q](https://eko-support.eko-instruments.com/hs-fs/hubfs/EKO%20Q%20Logo%201-2.png?height=75&width=248&name=EKO+Q+Logo+1-2.png)](https://eko-support.eko-instruments.com/knowledge-base?hsLang=en) EU | A | あ ⌄ English Deutsch Português Español [EKO website](https://eko-instruments.com) [EKO Q](https://eko-q.eko-instruments.com) [Contact us](https://share-eu1.hsforms.com/1MNfi6yWgQ6ab174lgwH9XA2eqp93) Search # How can we help you? Your browser does not support the video tag. 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[Detailed Report and plots](https://eko-support.eko-instruments.com/knowledge-base/understand-reports-and-plots?hsLang=en#detailed-report-and-plots) # Read and Understand the Carpet Plot The carpet plot is a useful diagnostic view in the Data Quality Report and is also available in Data Preview before running the analysis. It brings the complete irradiance dataset together in one image, making it easier to see whether the measurements are physically realistic for the location and season, whether recurring issues are present, and whether data is available throughout the expected measurement period. Before looking at calibration, orientation, or agreement with reference data, the carpet plot helps confirm that the dataset is fundamentally sound. Many serious problems can be identified quickly, often before any numerical test is evaluated. Problems seen here can affect the interpretation of the entire report. **What to look for in the carpet plot** Start by checking whether the annual and daily patterns are plausible for the location and measurement plane. The irradiance band should generally follow the expected sunrise and sunset boundaries, and the value range should be realistic for the season and type of measurement. Also check whether any gaps match known downtime or operating schedules, whether nighttime values remain close to zero without unexplained spikes, and whether any morning-to-afternoon asymmetry agrees with the installation geometry and known site conditions. #### What is the Carpet plot The carpet plot displays all measurements in a single image. The horizontal axis represents the progression of days through the year, typically from January to December. The vertical axis represents the time of day, usually from midnight and morning at the bottom to afternoon and midnight at the top. Each point is colored according to the irradiance measured at that moment, with darker colors representing lower irradiance and brighter colors representing higher irradiance. The color bar indicates the irradiance scale, normally in W/m². Its limits may differ between datasets, so colors should not be compared directly across examples unless the color-bar limits are identical. ![image 514](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20514.png?width=670&height=335&name=image%20514.png)**Healthy annual and diurnal patterns** When the data is healthy, daytime irradiance forms a clear band that begins near sunrise, reaches its highest values around the middle of the day, and decreases toward sunset. Outside daylight hours, the plot remains mostly dark because solar irradiance is close to zero. The annual shape depends strongly on latitude. At mid and high latitudes, the daylight band becomes wider in summer and narrower in winter. Near the equator, seasonal variation in day length is much smaller. Weather, cloud cover, rainfall, atmospheric conditions, and the measurement-plane orientation can still change the intensity, symmetry and texture of the daytime band. ### **Northern country** In a Northern Hemisphere location at mid or high latitude, daylight duration is shortest around the beginning and end of the year and longest around the middle of the year. For horizontal measurements, clear-sky midday irradiance is often higher in summer, although actual values also depend on weather and atmospheric conditions.![image 502](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20502.png?width=670&height=227&name=image%20502.png) #### **Tropical Southern Hemisphere country** In tropical Southern Hemisphere locations, the seasonal change in daylight duration is usually smaller than at higher latitudes. The annual pattern is seasonally reversed relative to the Northern Hemisphere: summer occurs around the beginning and end of the calendar year, while winter occurs around the middle of the year.![image 503](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20503.png?width=670&height=229&name=image%20503.png) #### **Near-equatorial country** Near the equator, day length remains relatively constant throughout the year. Seasonal changes in cloud cover, rainfall, aerosols, and other weather conditions may still produce visible differences in irradiance. The width of the daylight band should remain broadly stable, while the color and texture may vary. ![image 504](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20504.png?width=670&height=224&name=image%20504.png) #### **Time-related issues on the carpet plot** One of the first things to check is whether the measurement data respect realistic sunrise and sunset boundaries. In a consistent dataset, irradiance should appear mainly between sunrise and sunset, and these boundaries should shift smoothly through the year. Significant irradiance outside the expected daylight period, a systematic vertical displacement of the daytime band, or an abrupt shift partway through the year may indicate a mismatch between the measurement timestamps and the solar-position calculation. Possible causes include an incorrect time zone, a wrong UTC offset, missing or incorrect daylight-saving-time handling, logger clock changes, incorrect site coordinates, or timestamp conversion during data preparation. A carpet plot can show that the time reference is inconsistent, but it does not always reveal the exact cause. The site coordinates, time zone, UTC offset, logger clock, exported timestamps, and Daylight Saving Time settings should therefore be checked together. ![image 505](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20505.png?width=670&height=211&name=image%20505.png) ![image 506](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20506.png?width=670&height=218&name=image%20506.png) #### **Daylight Saving Time** The plot below is displayed in local civil time at a location that observes Daylight Saving Time. At the clock-change dates, both the measured irradiance pattern and the calculated sunrise and sunset lines shift by one hour. A visible discontinuity is therefore expected when a full year is displayed in local civil time. A configuration problem is more likely when only the irradiance data shifts, only the sunrise and sunset lines shift, or the two change on different dates. When both remain in UTC or fixed standard time, no daylight-saving-time discontinuity is expected. ![image 507](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20507.png?width=670&height=335&name=image%20507.png) #### **Abrupt changes during the measurement period** Abrupt changes in the daily pattern can indicate that the logger configuration, time zone, clock, or data-processing method changed during the measurement period. The same type of discontinuity may also be introduced while saving, transferring, merging, or preparing the data.![image 508](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20508.png?width=670&height=447&name=image%20508.png) #### **Gaps on the carpet plot** Missing data is particularly easy to identify in a carpet plot. Gaps may appear as blank or specially colored regions, depending on the plotting configuration. Vertical gaps indicate missing data over a continuous period, while horizontal or irregular gaps can indicate that data is missing only at particular times of day. Do not assume that every dark region represents missing data: valid nighttime values are also displayed near the bottom of the color scale. The carpet plot does not determine whether a gap is acceptable, but it shows its timing, duration, and regularity so that it can be compared with known maintenance, communication loss, power availability, or operating schedules. ![image 513](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20513.png?width=670&height=335&name=image%20513.png) ![image 509](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20509.png?width=670&height=281&name=image%20509.png)![image 512](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20512.png?width=670&height=335&name=image%20512.png) #### **Nighttime data on the carpet plot** At night, irradiance values should be close to zero, but not necessarily exactly zero. Thermopile pyranometers measure an energy balance, and small negative nighttime values are normal and expected. In some datasets, negative values are clipped or rounded to zero during logging or processing. EKO strongly recommends preserving the original measured values during both nighttime and daytime because they can simplify later analysis and troubleshooting. Extended periods of exact zero may reflect clipping, logger resolution, sensor output format, or data processing and are not automatically a sensor fault. Bright or noisy nighttime patterns, large spikes, or isolated non-zero blocks outside daylight hours deserve attention. Possible causes include wiring or grounding issues, logger errors, timestamp problems, electrical interference, or data manipulation. ![image 510](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20510.png?width=670&height=210&name=image%20510.png) #### **Noisy daytime data on the carpet plot** Cloudiness often makes a carpet plot look irregular or noisy during the day. This is normal and should not automatically be treated as a data-quality issue. Clouds introduce rapid changes in irradiance, which appear as patchy, streaked, or uneven coloring. The carpet plot is not intended to prove smoothness or measurement accuracy. Its purpose is to show whether the overall structure of the data is physically plausible. A noisy-looking daytime band can be healthy when it remains within expected daylight hours and is consistent with weather conditions.![image 514](https://eko-support.eko-instruments.com/hs-fs/hubfs/image%20514.png?width=670&height=335&name=image%20514.png) #### **Symmetry and plane-of-array orientation** For a horizontally mounted sensor with an unobstructed horizon, the annual daytime pattern is normally broadly centered around solar noon. For plane-of-array measurements, asymmetry may be expected and should be consistent with the azimuth and tilt of the installation. An east-facing plane normally shows a stronger morning response, while a west-facing plane normally shows a stronger afternoon response. Unexpected asymmetry may indicate shading, incorrect orientation, a time offset, or site-specific horizon effects. Uniform soiling usually reduces irradiance more generally and does not necessarily create a clear morning-versus-afternoon shift. The carpet plot can reveal clear orientation patterns, such as an east- or west-facing measurement plane, but it is not intended to quantify small azimuth or tilt errors. #### **Carpet plot as an early consistency check** The carpet plot appears early in the report and is also available in the EKO Q Data Preview because problems visible at this stage can invalidate later automated conclusions. If data is shifted in time, assigned to the wrong time zone, or missing over large periods, advanced analyses may produce misleading results. Correcting these issues and rerunning the analysis often resolves several warnings at once. Use the carpet plot as the first visual check. If the day-night pattern is correct, seasonal changes make sense for the location, gaps are understood, and nighttime behaviour is plausible, the dataset is likely suitable for deeper analysis. If not, the carpet plot has already fulfilled its purpose by showing where attention is needed before proceeding. The carpet plot also has limits. It does not provide a precise assessment of calibration accuracy, small orientation errors, or subtle shading effects. These questions are addressed later in the report using more targeted analyses. The purpose of the carpet plot is validation of basic consistency and physical realism at a large scale. #### **A practical final check** A healthy carpet plot shows an irradiance band that remains mainly between sunrise and sunset and changes smoothly through the year in a way that is plausible for the site latitude. Missing periods should have a known explanation, and the color scale should be reasonable for the sensor and measurement type. Nighttime readings should stay close to zero, allowing for small negative thermopile values, and any asymmetry should make sense for the measurement plane, horizon, and installation. A fixed displacement, abrupt change, unexplained gap, or bright nighttime block should be investigated before continuing with the rest of the analysis. The carpet plot is part of the Data Integrity section of the report. 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