Using SRTM30 height data

[mag]

Hi all!

I am creating a small java terrain viewer where I want to download height files(SRTM30plus) and match them with image files. Both files are downloaded from the same http://worldwind25.arc.nasa.gov server.

I am getting confused about the different lod-levels on image and height files. It seems that the image files have a maximum of 5 levels, and height files have more levels (9?). If this is the case, how do I find the correct height file for an arbitrary image file?

Any help will be appreciated!

[patmurris]

It is likely that you will not have matching tiles for elevations and images - each layer has its own quadtree grid with different zero level tile size, number of levels or extent.

What all virtual globes do is they download all the intersecting tiles for a given sector, at the needed or available resolution (level) for each layer involved (terrain, images, vector data, place names...) and recompose the scene using bits and pieces from each. This can involve resampling of some data.

[mag]

Thank you so much for your response!

I have been looking/running the javascript (and excel script) that was attached in this thread

http://forum.worldwindcentral.com/sh...nverter&page=4

and was given the impression that one .bil file matched a corresponding image file. I have successfully build my own view-dependent quad-lod algorithm that displays NWW imagery from the server, and now I want to add elevation data to these images.

If the two file-types do not cover the same geographic area, how do I find the correct .bil file(s) for an image tile?

Are you aware of any documentation or code available for this problem?

Your help is very much appreciated!

[patmurris]

You have to find out which level of detail you need, then which 'rows' and 'columns' are intersecting the sector you need.

See these documents:

NASA WorldWind Tile Structure
http://www.ceteranet.com/nww-tile-struct.pdf

http://issues.worldwind.arc.nasa.gov...le+systemt.gif

[mag]

Thank you for your all help!

I am already using this successfully for downloading my images. The problem arise when I am using the same technique for .bil files. The .bil files with the same level of detail and same "row" and "column" does not look right when using it together with my image.

I have attached two screenshots from my application which illustrates my problem. The height data does not look correct, and I have problems with cracks between adjacent tiles, even with tiles with the same LOD level.

I am just using the one corresponding .bil file and not recomposing the scene by using bits and pieces from several .bil files, this might be my problem.

[patmurris]

Originally Posted by mag

I am already using this successfully for downloading my images. The problem arise when I am using the same technique for .bil files. The .bil files with the same level of detail and same "row" and "column" does not look right when using it together with my image.

The level, row and cols for the SRTM data is likely different then from the image layers. You have to determine them separately for each layer in fact. This essentialy depends on the layer level zero tile size (or delta) in degrees, and the tiles dimensions (512x512 for images, 150x150 for .bil).

The cracks you get are completely expected with tiled terrains - welcome to virtual terrains In WW (.net and java) the tesselator builds 'skirts' on each terrain tile side to fill those gaps.

Are you rewriting WW?

[rational_8]

Hi,
In my understanding bil file need to be of same width-height ratio only. It does not need to be for all tiles. what is the relation of bil file size with layer level zero tile size. any formula? I am actually looking for importing my custom tiles and .bil file into WW. Any help will be great.

[patmurris]

There is no constraints regarding how fine the SRTM data layer (.bils) is compared to other image layers. Each layer has its own level parameters that describe how its tiles are organized. WW will determine which level and which row and cols need to be involved from each layer when compositing the terrain.

[rational_8]

Hi patmurris,
Thanks for response. I understand the NASA WorldWind Tile Structure you had posted above. But where I could found details of level parameter description so I could create my own .bil files according to my texture data tiles.

[patmurris]

Look into globe.ElevationModel, EarthElevationModel...