VIRTUAL EARTH PRODUCTS

This page contains some comments about virtual earth products that are currently available. The comments are mainly limited to those issues that are relevant to this site. They should not be regarded as general reviews.

See also my Screenshot Comparisons Page.

To summarise: I would like to have an earth viewing service that will both (a) show me a high resolution image of my home (note to Google and NASA: it's at 56°20' N, 3°15' E) and a decent 3D image of Mount Everest (note to Microsoft: it's at 27°59' N, 86°55' E, on the Tibet-Nepal border).

Note: The paragraphs that follow contain several links to Wikipedia articles. Readers with Google Earth can click on the coordinates in some of these articles, then click on "Find this location" with Google Earth.

This page was last updated on 24 June 2007, but most of it was researched and written earlier. If any reader finds more recent imagery or terrain updates that are relevant to this page, please tell me!

For much of the world, this is a good product. It is global, and its images have been accurately placed. and, most important of all, it is free. It can be downloaded directly from here.

The primary source of imagery has a resolution of 15 metres and is from NASA's global Landsat program. For some areas, including most of the USA, many cities, and mountains including Mount Everest and most other Nepalese mountains over 8000m, there are higher resolution images.

Google Earth images can be tilted to give realistic three dimensional views. But these views require DEM data. Prior to 23 November 2006, the only sources of DEM data that they were using were USGS, GTOPO30 and SRTM. The USGS source is good but only covers US territory. The quality of GTOPO30 is variable and its resolution of 30 arc seconds is not adequate. The quality of SRTM is almost uniformly good and its 3 arc second resolution is adequate, but it mapped nothing north of 60°N or south of 56°S, and missed many areas of very high relief. This spoiled the 3D images of thousands of mountains, including Matterhorn and Everest. However, GE have now incorporated DEM data from alternative sources, and the majority of mountains are now much better. See the Ogle Earth pages posted on October 18 and November 23.

But there are still issues. Surprisingly, the 3D images of Canada are still GTOPO30/SRTM, despite freely available data from the Canadian government. Many mountains in Western Canada are adversely affected; Monarch mountain is a prominent example. Also, when GE made the above mentioned November 2006 terrain update, they placed the new elevation data 3 arc seconds (one pixel, about 90 metres) too far north. This can be verified by homing on on water bodies surrounded by slopes (e.g. Norwegian fjords) and moving the cursor slowly in a north-south direction. Where Landsat imagery has been used, the effects are not seriously noticeable, but where higher resolution images have been used, dark shadows appear along steep strips which the 3D images show to be the upper parts of south facing slopes, but which in reality are on the north facing sides of narrow ridges. See Everest, especially its north east ridge. There are also occasional issues related to the accurate placement of these high resolution images. Most are well placed but there are exceptions. The image covering Annapurna I has been placed about 10" too far north and 8" too far east. This spoils the close-up 3D view. To verify that the main issue here is image (not DEM) displacement, move along the ridges to the east and south, to where the hi-res image merges with the surrounding Landsat imagery, and note the relative discrepancies. If, as I hope they will, especially following their acquisition of Endoxon, GE incorporate more hi-res imagery, then I think that they need to fix these issues.

For the future, it would be nice to see an option to reduce the angle of the field of view but increase the magnification. This would permit the identification of features of long distance views, providing a kind of dynamic version of my panoramas page. For this, it would be necessary to allow for the curvature of the earth. I do not know if GE or WW do this at present, but if not, there may be an adverse effect on response time.

Google Earth is sometimes regarded as a product that competes successfully against NASA World Wind (see below). But it should be pointed out that without NASA's public domain imagery and SRTM DEM data, Google Earth would be next to nowhere, other than in the USA and Switzerland (see below)

Update 2 March 2007. While I welcome the addition of Swiss 50cm imagery and 10m terrain, I am very disappointed to note several flagship mountain summit errors. Here are two screenshots; the first is of the Dom, whose 4545m summit had been reduced to a flat plateau that gets no higher than 4440m. The second is of the Matterhorn, which shows a summit ridge that is too long and with spurious summits at either end. The Italian and Swiss summits should be closer together than these, with a slight drop in between, along a steep ridge, not a flat area. The view is from the south and shows overhanging shadows, showing that there is still terrain/imagery misalignment. There are several other summit inaccuracies in this area, including Weisshorn. There is also an issue around Tödi (46°49'N 8°55'), where an area rising to 3614m has been flattened to a 3516m plateau. Most other parts of Switzerland that I have checked seem to be OK, although I do not find significant improvements over the Swiss 1" data here.

NASA World Wind

NASA World Wind can be downloaded from here. It is, in many respects, similar to Google Earth, see above. Much of the content of the GE review also applies to WW. But here are some differences:

* I find that the WW controls are less user friendly than those of GE, and tend to respond slower, although I am open to suggestions re how I might better use them.
* There are fewer, if any, high resolution mountain images on WW.
* WW's underlying DEM has, like that of GE, been upgraded from alternative sources (to "SRTM+"). But the upgrade took place on 20 April 2006, and as far as I am aware there have been no upgrades since. Consequently, at that time of writing (26 December), the extent of WW's accurate DEM coverage is lagging behind GE. But most well known mountains are still covered, and there does not appear to be the 3" displacement that there is in GE. There are still some overhanging shadows, due to the variable quality of the alternative sources in some areas, and the greater contrast of WW tends sometimes to show them up more, but they are narrower, and they are not amplified by the high resolution imagery that GE has. But note that, to the best of my knowledge, SRTM+ only works with versions 1.3.4 or more recent; users who have not downloaded WW updates since April may get the old and woeful 3D mountain images.
* WW's software is open source, a fact that NASA make much of. I agree strongly with open source policy. But as I am not into the languages they use, and only a small number of users are into digging into the structure of the software, the benefits seem to me to be more limited that NASA imply.
* When I tilt the WW images, there seem to be issues with the accuracy of the information displayed. The positions of the latitude and longitude grid lines appear to move, and to obtain accurate coordinates I need to move the image rather than the cursor. I would also prefer to see the coordinates in DMS format rather than decimal degrees, but I cannot figure out how to get this.

Microsoft Virtual Earth

Microsoft Virtual Earth is another earth viewing service which is provided free to the world community. It can be downloaded via Live Search. Here are the main differences between VE and Google Earth.

* VE has a facility which allows tilting and rotating using the mouse with the control key depressed. This is a significant improvement; I find this facility to be user friendly.
* Comparing the extent of high resolution imagery, for much of Europe, VE seems to have a significant lead, see Lochmill. For Europe, VE also has better coloring, although when I checked Colorado I found VE had greyscale images. In the Himalayas, Google Earth leads in the hi-res image department.
* For most of the world, VE's underlying terrain model is based on the same NASA SRTM model that is used by GE and WW. But here is the big difference: where there is no SRTM data, VE are still mainly interpolating or using low resolution data. For some parts of Europe, including the Alps, there is evidence of input from another 3" source, but for mountain areas, the quality of this source is not good. A tour of the polar regions (including most of Norway and Sweden) and SRTM mountain blackspots like Matterhorn and Everest continues to shows up mountain eyesores that were, until recently, visible on Google Earth. Yuk.
* I don't think there is any open source software. That's not the Microsoft way.
* VE has underlying road map coverage, which can be used in conjunction with aerial coverage. This is a great help for finding places. But against that, VE does not display coordinates or terrain heights, at least not as far as I have been able to figure out. While this may not be an issue for the majority of users, it is for me, and I cannot see why this should not at least be made an option.

Other earth viewing services

There are other earth viewing services. As far as I am aware, they either do not permit 3D viewing or, if they do, they often use poor quality DEM data, but any reader who thinks I have missed something should tell me. There follows a review Microsoft Flight Simultator, see below, which does not use good default DEM data, but it permits third party add-ons and can be made comparable with the above products. The comments below are generally equally applicable to FS2004 and FSX (which was released in late 2006), although for some areas FSX offers better default terrain mesh data.

Microsoft FS, in its basic form, is primarily designed for aircraft pilots and would-be pilots to practice on, and its default scenery and topography, which is based on landclass data rather than images, is poorer than that of GE, NWW and MVE. Its main advantage, however, is that it allows independent developers to create and add on their own scenery meshes. This facility has been taken up, with the use of data downloaded from my DEM page; the aerial view of the Karakoram 8000ers created can be viewed here and compared with the pre-update (see above) Google Earth view. The FS and pre-update GE views of Birutaso, a remote 6691m mountain in eastern Tibet, can be compared with an actual photograph (external link).

A problem with some flight simulator meshes is that they only provide elevation data, and this can be inconsistent with Microsoft's default coastline and lakes data, causing false beaches and cliffs. The problem is acute in Norway, but fortunately compatible coastline has been provided by a new product called Ultimate Terrain Europe. Users of accurate Scandinavian mesh data should also download a UT patch to make the lake elevations compatible with that data; otherwise lake towers and pits will spoil the scenery. There is similar problem with the Faeroe islands, which UT Europe does not cover, but a good Faeroes mesh which includes compatible coastline ans many other local features can be downloaded here.

I have not been able to figure out how to identify (other than via grid references) the features displayed in FS imagery; this is a serious disadvantage relative to GE/WW.

FSX update. I have just carried out more checks on the FSX default mesh. For most of the world, the FSX default mesh is no better than the FS2004 default mesh, but for most of North America, Europe, and a few other areas, the use of SRTM and other high resolution data has brought about a substantial improvement. For Norway and Sweden, where there is no SRTM data north of 60°21", FSX uses accurate meshes which seem to have been created from national survey data. But for most other areas, including most of the Alps, the SRTM no data areas have been filled by crude interpolation. For some screenshot comparisons, see FSGlobal2008.

This is another similar product with great potential, although at present it is only available for Italian territory. It can be accessed here. Note that it is check the box above "Avvia ItalyeXplorer" and if necessary download the Blaxxun plug-in (click on "questo file"). The photo-realism is good. Prior to November 23, the 3D accuracy was better than Google Earth but, at the time of writing, limited. See some Matterhorn screenshot comparisons: pre-update GE(joke), IE, and FS, clearly showing the profile of the western ridge.

Another database from Italy Explorer provides even better coverage of the Rimini area. Another Italian product, from Geomind, gives excellent coverage of Tuscany.

ASTER Digital Elevation Data

ASTER digital elevation models are derived from stereo pairs of images with a resolution of 15 metres. They are widely claimed to be a good source of international elevation data, and they can be ordered from here. There is more information about ASTER data from the USGS and from terrainmap. Some ASTER data can be downloaded from GLCF Maryland.

Before May 2006, Aster data used to be free on-demand. Then they changed their production methods and ceased to make their former archives available. Their new product is available but it is not free. I have not seen it, so I do not know if it is a significant improvement. The comments below refer to the old product.

ASTER data is not worldwide, but many high relief areas are well covered. An article by a Japanese author claims ASTER data will enable there to be a DEM covering all of Asia with a resolution of 30m and an SD error margin of about 8 metres. Sounds too good to be true! The USGS site mentions errors of 7-50m.

About two years ago I obtained some AST14DEM tiles for the High Alps and studied them carefully. They are not calibrated against ground control points.

For areas of high relief, it is hard to see how these tiles could be used to create anything near to meeting the above accuracy claims. Careful examination suggests that the 30m resolution is not a true resolution; it is easy to create a 200m DEM, then interpolate to create a so-called 30m DEM. There are large voids due to both cloud cover and high relief terrain. But more seriously, there are large random wells and spikes that frequently attain a height or depth of 500 metres or more; on one tile I found a 1500m spike. The effect is to make a color-relief image look as though paint has been spilled onto it. And even if these could be removed, there would still be very large underlying inaccuarcies. It is claimed that better DEMs can be obtained by better processing of images than is carried out for AST14DEM tiles, and this claim would seem to be credible, but it still seems to me to be very unlikely that ASTER tiles could be used to create DEMs more accurate than SRTM or local topographic mapping.

SRTM has a resolution of 3", which is well within the practical limit of AST14DEMs, and comparison with local topos suggests that SRTM is very accurate, with errors typically much lower than the "16m 90% confidence" it claims. So ASTER is only really of interest north of 60°N or south of 56°S, or for areas of high relief which SRTM was not able to map.

There is no need for ASTER coverage of Alaska or Northern Canada. More accurate DEMs from local topographic maps can be downloaded from US and Canada government websites. Very little of Greenland and Antarctica has been covered by ASTER, and even if it were, the images of ice caps and snow fields would be useless for DEM production. Norway and Sweden are covered by DEMs based on Russian 100k topos downloadable from this site, and similar DEMs can and will be created for the rest of the Arctic. So other than for those areas of very high relief where SRTM is void, ASTER would seem to be of very little consequence, and it is therefore in such high relief areas that ASTER data should be judged. My judgment of all the old ASTER data that I have seen for SRTM void areas is that it fails to match the quality of topographic maps. I have not yet seen any new data (see above) although I have placed an order for a granule for evaluation.

Therefore I have not, so far, used ASTER data to fill SRTM no data areas, although I may consider it where there is no accurate topographic data and the quality is OK. So far I have not seen any evidence that Google Earth, CGIAR or even NASA World Wind have opted for ASTER data as an alternative source.