FAQ about HelioClim Databases of Solar Radiation

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• Q1_ I have ground measurements together with HelioClim data within a common period. What is the best way to fuse them?
• Q2_ I have data from two different HelioClim databases within a common period, typically HC-1 and HC-3, with a common period 2004-2005. What is the best way to fuse them?
• Q3_ Why two requests inside the same pixel MSG return 2 different irradiation values?
• Q4_ I tried to launch a service, and I got the message error on the top of the page:"unable to write map image". What is the problem and what should I do?
• Q5_ I think I made two identical requests, and I retrieve two different results, why?

Q1. I have ground measurements together with HelioClim data within a common period. What is the best way to fuse them?
A. If the measurements are well made with a well calibrated instrument, the ground measurements (GRD) are supposed to better represent the reality of a site than HelioClim since the measurement is more local. The principle is to adjust HelioClim data onto ground measurements. We provide a service of calibration of the HC onto GRD data More Info.

Q2. I have data from two different HelioClim databases within a common period, typically HC-1 and HC-3, with a common period 2004-2005. What is the best way to fuse them?
A.Various validation experiments between HC databases and ground measurements have shown that HC-3 data are closer to actual values than HC-1. Thus, HC-3 data have less uncertainty than HC-1. The principle is to adjust HC-1 data onto HC-3 data for the common period. One way to do that is to compute a regression line between HC-1 and HC-3 data. One may use least-square method or alternatively the first axis of inertia method. Then, the relationship may be applied to the whole series of HC-1 data, thus offering a consistent data set from 1985 till nowadays.

Q3. Why two requests inside the same pixel MSG return 2 different irradiation values?
A. The HelioClim databases consist of Global Irradiance values over the horizontal plane. These values take into account a default value for the elevation, which is the values of TB5' (Terrain Base with 5 min of spatial resolution, i.e. 10 km). Several years ago, new elevation databases with higher spatial resolutions became available: GTOPO30'' (1 km) and finally SRTM (90 m) available worldwide. Two requests made in the same pixel return different results since a correction with the height of the high resolution elevation database SRTM. A webservice of elevation (TB5, GTOPO and SRTM) is available here.

Q4. I tried to launch a service, and I got the message error on the top of the page:"unable to write map image". What is the problem and what should I do?
A. This error message means that your output temporary file has been deleted by mistake. This bug has normally been corrected. But if you have this error message, please contact us so that we can fix it as soon as possible.

Q5. I think I made two identical requests, and I retrieve two different results, why?
A. Different versions of HC3:The most frequent case is when you select the version 2 of HelioClim3 instead of version 3. The differences stem from a the use of different method of split. More precisely, the new version HC3v3 uses a more recent split model which leads to a more important value of the Beam (also named Direct) Horizontal Irradiation, and a smaller value for the Diffuse component. As a consequence, as the components of the irradiation over the inclined and normal planes are also different. Globally, these components are higher than for version 2.
Different elevation values: you should also check the altitude value. If you set the elevation at its default value (-999), the algorithm is computing the elevation value in the SRTM database. As a consequence, the returned SRTM value can be different from the one you would have entered yourself as input parameter.

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