Goals
- Facilitate to assess the spatial misregistration between OLCI and SLSTR bands. The misregistration can be assessed in two ways: In the first way, SLSTR L1B (FR) products are compared to SLSTR L1B products. In the second way, bands from a SYN L1C product are compared.
Background and strategic fit
By this manner, the Synergy processing can be validated.
Assumptions
Either OLCI L1B FR and SLSTR L1B or SYN L1C product available.
Requirements
| # | Title | User Story | Importance | Notes |
|---|---|---|---|---|
| 1 | Assessment using reference points | The user opens an OLCI L1B and a SLSTR L1B product. He/she continues to load in a list of reference points with well-known geolocations. These points are placed on the image grids. The user can then assess how well the points were placed. The primary format for the geolocations are plain text, CSV files: <lat>, <lon>, <label>. Useful are also Google Earth KMZ and ESRI Shapefiles. | MUST |
|
| 2 | Assessment by comparing geolocations | The user places a pin in an OLCI L1B product at some distinct feature (e.g., the tip of an island). The user places another pin at the same feature in a SLSTR L1B product. Both pins will have geolocations assigned to them. These geolocations can be compared. | COULD | This would also require form of uncertainty information: What is the error which is introduced by the application of the inverse geocoding? |
| 3 | Assessment by transferring pins | The user places a pin in an OLCI L1B product. A pin is automatically set in a SLSTR product on base of the pin's geolocation. The displacement of the pin and its expected position can be assessed visually. In addition, the geocoding from the expected position in the SLSTR product can be derived and compared to the pin's geoposition. | MUST | This would make use of a global or synchronized pin. |
| 4 | Assessment in a SYN L1C product | First, the L1C "OLC_RADIANCE_O17" band (which is located at 865 nm, the reference wavelength) is opened. The user places pins in one of the camera images or imports pins from an external file. The "OLCI pins" are automatically transferred to "MISREGIST_SLST_NAD_Oref_S3" (S3 is roughly at 865 nm). This is a nc-file which contains two variables: "row_corresp" and "col_corresp" for each camera image. From these variables, the pixel position in "SLST_NAD_RADIANCE_S3" can be retrieved. The position of the pixels can be compared visually or by comparing the geo-locations. | MUST |
User interaction and design
For Requirement 2:
Define how to visualize displacement uncertainty. Using the displacement bands could be a start.
Also, consider the following formula where GC is a geocoding, G is a geolocation, and P is a position in image coordinates:
G = GC_olci (P_olci)
P_slstr = GC^-1_slstr (G)
G' = GC_slstr (P_slstr)
P'_olci = GC^-1_olci (G')
delta_G = G - G'
delta_P_olci = P_olci - P_slstr
delta_G and delta_P_olci would then be displacement measures.
Another attempt to be used on SLSTR or OLCI grid:
- Determine four corners and center of a pixel
- Determine the resulting five geolocations
- From these, determine relative displacement
For Requirement 3:
Use a global/synchronized pin. This pin would be placed in one product, and then, based on either its geoposition or its image coordinates, be placed in all (or some) other products which are currently opened in the Sentinel-3 Toolbox. There could already be uncertainty information attached to the pin ("uncertainty pin"), maybe in the form of an ellipse. See "For Requirement 2" for ideas on how to arrive at this information.
Also, there is the open issue how such a synchronized pin shall be saved.
For Requirement 4:
Requirement 4 consists of two phases: Transferring a pin to another grid ("OLC_RADIANCE_O17" -> "MISREGIST_SLST_NAD_Oref_S3") and then, based on the information at the "MISREGIST_SLST_NAD_Oref_S3", placing it in "SLST_NAD_RADIANCE_S3". These steps could easily be automated.
When the case for other olci or slstr bands is considered, the procedure is different in that the "MISREGIST_OLC" bands are required.
Questions
Below is a list of questions to be addressed as a result of this requirements document:
| Question | Outcome |
|---|---|