The bigBed format stores annotation items that can either be simple, or a linked collection of exons, much as BED files do. BigBed files are created initially from BED type files, using the program bedToBigBed. The resulting bigBed files are in an indexed binary format. The main advantage of the bigBed files is that only the portions of the files needed to display a particular region are transferred to UCSC, so for large data sets bigBed is considerably faster than regular BED files. The bigBed file remains on your web accessible server (http, https, or ftp), not on the UCSC server. Only the portion that is needed for the chromosomal position you are currently viewing is locally cached as a "sparse file".

Additional indices can be created for the items in a bigBed file. These indices can be used to support item search in track hubs. See Example 3 for an example of how to build an additional index.

See this page for help in selecting a graphing track data format that is most approriate for the type of data you have.

Note that the bedToBigBed utility uses a substantial amount of memory; somewhere on the order of 1/4 times more RAM than the uncompressed BED input file.

To create a bigBed track, follow these steps:

1. Create a BED format file following the directions here.
   • When converting a BED file to a bigBed file, you are limited to one track of data in your input file; you must create a separate BED file for each data track.
   • Your BED file must be sorted by chrom then chromStart. You can use the UNIX sort command to do this: sort -k1,1 -k2,2n unsorted.bed > input.bed
   • This is the file that is referred to as input.bed in step 5 below.
2. Remove any existing 'track' or 'browser' lines from your BED file so that it contains only data.
3. Download the bedToBigBed program from the directory of binary utilities.
4. Use the fetchChromSizes script from the same directory to create the chrom.sizes file for the UCSC database you are working with (e.g. hg19). Note that this is the file that is referred to as chrom.sizes in step 5 below.
5. Create the bigBed file from your sorted BED file using the bedToBigBed utility like so: bedToBigBed input.bed chrom.sizes myBigBed.bb
   
6. Move the newly created bigBed file (myBigBed.bb) to a http, https, or ftp location.
7. Construct a custom track using a single track line. Note that any of the track attributes listed here are applicable to tracks of type bigBed. The most basic version of the "track" line will look something like this:
   track type=bigBed name="My Big Bed" description="A Graph of Data from My Lab" bigDataUrl=http://myorg.edu/mylab/myBigBed.bb
8. Paste this custom track line into the text box in the custom track management page.

Example One

In this example, you will use an existing bigBed file to create a bigBed custom track. A bigBed file that contains data on chromosome 21 on the hg19 assembly has been placed on our http server. You can create a custom track using this bigBed file by constructing a "track" line that references this file like so:

Include the following "browser" line to ensure that the custom track opens at the correct position:

Paste the "browser" line and "track" line into the custom track management page for the human assembly hg19 (Feb. 2009), then press the submit button. On the following page, press the chr21 link in the custom track listing to view the bigBed track in the Genome Browser.

Example Two

In this example, you will create your own bigBed file from an existing bed file.

• Save this BED file to your machine (this satisfies steps 1 and 2 above).
• Save this text file to your machine. It contains the chrom.sizes for the human (hg19) assembly (this satisfies step 4 above).
• Download the bedToBigBed utility (see step 3).
• Run the utility to create the bigBed output file (see step 5):
  bedToBigBed bedExample.txt hg19.chrom.sizes myBigBed.bb
• Place the bigBed file you just created (myBigBed.bb) on a web-accessible server (see step 6).
• Construct a "track" line that points to your bigBed file (see step 7).
• Create the custom track on the human assembly hg19 (Feb. 2009), and view it in the genome browser (see step 8). Note that the original BED file contains data on only chromsome 21.

Example Three

In this example, you will create your own bigBed file from a fully-featured existing BED file that contains the standard BED fields up to and including the color field (field 9), plus two additional non-standard fields (two alternate names for each item in the file). BigBed files can store extra fields in addition to the predefined BED fields. If you add extra fields to your bigBed file, you must include a .as (AutoSQL) format file describing the fields. See this paper for information on AutoSQL. There are several sample .as files here. This example also demonstrates how to create extra indices on the name field, and the first of the extra fields to be used for track item search.

• Save this BED file to your machine (this satisfies steps 1 and 2 above).
• Save this text file to your machine. It contains the chrom.sizes for the human (hg18) assembly (this satisfies step 4 above).
• Save this .as file to your machine. The .as file contains a description of the fields in the BED file. This is required when the BED file contains a field for color.
• Download the bedToBigBed utility (see step 3).
• Run the utility to create the bigBed output file with an index on the name field and the first extra field: (see step 5):
  bedToBigBed -as=bedExample2.as -type=bed9+2 -extraIndex=name,geneSymbol bedExample2.bed hg18.chrom.sizes myBigBed2.bb
• Place the bigBed file you just created (myBigBed2.bb) on a web-accessible server (see step 6).
• Construct a "track" line that points to your bigBed file (see step 7). Because this bigBed file includes a field for color, you must include the itemRgb attribute in the "track" line. It will look somewhat similar to this (note that you must insert the correct URL to your bigBed file):
  track type=bigBed name="bigBed Example Three" description="A bigBed File with Color and two Extra Fields" itemRgb="On" bigDataUrl=http://yourWebAddress/myBigBed2.bb
• Create the custom track on the human assembly hg18 (Mar. 2006), and view it in the genome browser (see step 8). Note that the original BED file contains data only on chromsome 7.
• If you are using the bigBed file in a track hub, then you can use the additional indices for track item searches. See the setting "searchIndex" in the Track Database Definition Document for more information. For example, if you ran your bedToBigBed with the option of "-extraIndex=name", you will only be able to search on the name field by adding the following line, "searchIndex name" to the stanza about your bigBed in the hub's trackDb.txt file.

Sharing Your Data with Others

If you would like to share your bigBed data track with a colleague, learn how to create a URL by looking at Example 11 on this page.

Extracting Data from the bigBed Format

Because the bigBed files are indexed binary files, they can be difficult to extract data from. Consequently, we have developed the following two programs, both of which are available from the directory of binary utilities.

• bigBedToBed — this program converts a bigBed file to ASCII BED format.
• bigBedSummary — this program extracts summary information from a bigBed file.
• bigBedInfo — this program prints out information about a bigBed file.

Troubleshooting

If you get an error when you run the bedToBigBed program, it may be because your input BED file has data off the end of a chromosome. In this case, use the bedClip program here before the bedToBigBed program. It will remove the row(s) in your input BED file that are off the end of a chromosome.