FAQ
Spot 3D
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Q1 - Can I buy HRS stereopairs? The basic offer to access HRS data is a 3-layer database named Reference3D : one DEM layer, one orthoimage layer and one extensive quality meta-data layer. From our studies, very few applications really require the use of the stereopairs. Please contact us for any specific need. |
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Q2 - What is the orthoimage layer of Reference3D made of? What is its use? Reference3D orthoimage layer (1/6 arcsec sampling, ie 5m resolution on the Equator) is made by rectifying and mosaicking Panchromatic HRS images onto the HRS DEM. It should prove unrivalled for automatic location of optical imagery, due to its excellent absolute accuracy. Reference3D orthoimage can be used to extract the main networks (main roads, rivers,…) but is not really designed to perform cartographic works : SPOT HRV and HRG are far best choices for this purpose. |
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Q3 - Does Reference3D include a DEM or a DTM? By the way, what is exactly the difference between a DEM and a DTM? Usually, the acronym DTM refers to the altitude of the ground itself : a DTM would consider the soil under the buildings, and around the feet of the trees. On the contrary, a DEM includes the maximum altitude everywhere : roofs of the buildings, top of the trees (often known as “top of the canopy”). The processing of stereoscopic images (space or aerial) naturally produces a DEM, because neither the operator nor the matching software can see the ground itself through the roofs and leaves. If a DTM is really requested, a very heavy correction process has to be undergone, to rub out the trees and buildings, and by no way one can guaranty the accuracy of the result without field checking. For these economical and accuracy reasons, and also because the elevation information has its own value (eg for airplanes), Reference3D includes a DEM, not a DTM. |
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Q4 - What is the circular horizontal accuracy of Reference3D? The horizontal accuracy of Reference3D without GCP is 15 m @ 90%. Sophisticated processing tools are necessary to meet this goal. The above specification is fulfilled over large areas processed into a single block. Small areas will be processed through specific methods. For further information about Reference3D accuracy requirement, please refer to the “Reference3D Product Description” : |
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Q5 - Before HRS, the posting of the DEMs extracted from SPOT 1 to SPOT 4 Panchromatic was 20 m. Why was it changed to 1 arc second for Reference3D? Does that mean a loss in quality? Certainly not. HRS DEMs show far better accuracy than any other DEM extracted from medium resolution space data. The move to 1 arc second was made to comply with the DTED2 standard. Note that 1 arc second represents 30 m by 30 m on the Equator, but only 30 m by 21 m at a 45° latitude. |
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Q6 - Can I place an emergency order to task HRS? A long-term collection plans have been established to achieve the maximum coverage within HRS theoretical lifetime (5 years). Of course, the collection plan is always under revision, to cope with the needs of Reference3D customers. As of April 2004, the qualified HRS image archive already counts with more than 60 millions km² available for DEM extraction and Reference3D production. This amount should continue to grow with time. However, since HRS has no side-looking mirror, the revisit time is 26 days. This means that HRS cannot be tasked to address extremely urgent crisis needs. Please check the SPOT 3D page of the Technical Information area of our web site in order to find the HRS worldwide coverage map available for DEM extraction and Reference3D production. |
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Q7 - Is Reference3D 100% produced from HRS data? Reference3D is mostly derived from HRS, but not necessarily 100%. Since HRS revisit capability is only 26 days, we envision that the collection of absolutely cloud-free data over every acre of land could be very long in some areas. That is the reason why it is planned to complement Reference3D with external data : SPOT stereopairs, ASTER, SRTM (DEM layer only),… The quality layer will register the corresponding mentions. |
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Q8 - I am interested in Reference3D, but I need only HRS derived data, not the whole thing with external data complementation, …(eg because I will fill the gaps by myself). No problem. Our Reference3D contracts are flexible and fully customised. That is why we will need some time to consider all the technical solutions to address your exact needs (in the feasibility study step) before proposing you a commercial deal. |
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Q9 - About Reference3D delivery unit and price? Reference3D is delivered through a DTED frame (1° x 1° tile). The price of Reference3D is depending upon the type of licencing, and also the global extent of your area. |
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Q10 - DEM papers often refer to DTED. What does this mean exactly ? In support of military applications, the U. S. National Geospatial-Intelligence Agency (NGA) has developed DTED standard digital datasets (Digital Terrain Elevation Data). It is a uniform matrix of terrain elevation values, which provides basic quantitative data for systems and applications that require terrain elevation, slope, or surface roughness information. You can find some documentation about DTED on NGA’s Web site. DTED standards encompass several levels of accuracy, from DTED level 0 to DTED level 5. Level 0 content is equivalent to the elevation information of a 1,000,000-scale map (more or less equivalent to DCW, or GTOPO30, or GLOBE). DTED 1 has mainly been extracted from 250,000-scale maps. DTED 2 content is equivalent to the contour information represented on a 1: 50,000 map. Some DTED figures :
. ACCURACIES (@ 90%).
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Q11 - Is Reference3D the only 3D offer from Spot Image? Certainly not. Our offer is based on 3 different products:
Please check the SPOT 3D page in order to find the SPOT DEM worldwide coverage map (More than 70 M sq. km as of today). |
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Q12 - About SPOT DEMs delivery unit and price? SPOT DEM and SPOT DEM Precision are delivered through a tailored framing (per sq. km offer). The minimum order size is of 3,000 sq. km. The price of these products is detailed in our price list to be found on our web site. Should you need smaller AOIs, please contact us as we most certainly have a solution to your problem. [+] More |
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Q13 - Why did you choose EGM96 as Reference3D elevation system ? EGM96 was chosen for its reliability (produced by NASA and NGA) , its accuracy (defined in relation with WGS84), and its compatibility with SRTM and with DTED products. Furthermore, the EGM96 model is very commonly used among the GIS -and Earth Observation- user community. EGM96 posting is 15' x 15' (approx. 25km on the Equator). Please note that interpolation method within EGM96 is not really a key issue in this case, because the variations of the geoid values are smooth, as compared to the vertical accuracy required for Reference3D. Further technical information about EGM96 : [+] More |
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Q14 - How do you cope with different sea levels (tide effects) on adjacent images ? The tie points between adjacent HRS pairs are chosen on the ground. This gives a perfect coherency of the elevation values over the landmasses. Over the water areas (sea) the matching process is completely inefficient, and no elevation value can be computed. The sea is flattened to a 0 value within an interactive process. Therefore, when very different sea levels are involved, the effect can sometimes appear on the orthoimage along the seashore, as a sudden "break" of the shoreline. In this case, we try to manually choose a new connecting line for the adjacent orthoimages, to eliminate the discrepancy. Regarding the accuracy of the DEM in these areas, it should be noted that the maximum amplitude of sea tide is commonly far less than 10m (ie +/- 5m around the average) ; therefore, the Reference3D requirement can be met. |