Vector Datasets and Formats
Much vector data is stored in conventional GIS
formats, such as those of ESRI SHP or MapInfo formats. However,
there are a number of vector data formats which are specific to particular data
types and US agencies. These are important because they are the format in
which a large amount of public-domain data is distributed.
Global Vector Datasets
- VMAP. The US NGA has a global vector dataset
called VMAP. Read more about it below and on the
Data Openness page.
- Natural Earth
is a public domain map dataset in very low resolutions.
- It contains mostly cartographic features like political boundaries
which aren't much use for realistic terrain visualization.
- There are many free Shapefiles for global political boundaries, but
surprisingly they are all quite deeply flawed in many respects, as detailed
in the very impressive 4-part blog entry
Free Shapefile of Countries of the World
- "Digital Line Graph - Optional format"
- the USGS's older vector data format, which they are trying to phase
out in favor of
the (very bad)
- 1:100,000 DLGs are available free
online from the USGS for most of the USA, higher-resolution DLGs can
be purchased from the USGS
- DLG-O is much simpler to read than SDTS, with a file
naming scheme that's also much easier to use
- the format has some documentation
- the format does support specification of a Datum, although this is not
mentioned in the USGS spec!
- beware: pretty much all of the USGS data that says NAD27, can
actually mean OHD, NAD27 Alaska, NAD27 Aleutians, or NAD27 CONUS,
depending on where the data actually is.
- DLG layers are missing a lot of important information, including:
road widths, names of streets and rivers, bridges, and traffic direction
- common DLG layers:
- HYF - Hydrography
- MTF - "Miscellaneous Transportation Features" includes airfields
- RRF - Railroads
- RDF - "Roads and Trails"
- Road layers
- It's possible for a road to have no attribute codes, or many.
The data is highly variable. Most (like 99%) have only the
"road class" attribute, which doesn't tell you much. However, i
have encountered a single road with all of the following:
- 170 0 Photorevised feature
170 201 Primary route, class 1, symbol undivided
170 216 Arbitrary extension of line (join or closure)
170 607 Underpassing
170 654 Road width 86-95 feet, 0.045 inches at 1:24,000
171 4 Number of lanes = 4
174 17 State route number = 17
- source code for reading DLG files is part of the
Other Vector Formats
BTS (Bureau of Transportation Statistics)
- Part of the U.S. Department of Transportation
- Provides free geospatial data for highways, railroads, and all other
kinds of transportation, for all of North America. Historically,
this was in their own ASCII file format.
- SDTS is used for vector data as well as other data types
U.S. Military Formats
- (supposedly) unclassified military data from the NIMA Terrain Resource
- previously known as DCW (Digital Chart of the World)
- 3 levels of detail: VMAP0, VMAP1, and VMAP2
- all based on the VPF format (Military Standard Vector Product
- according to an experienced developer: "Don't try to read VPF
unless absolutely necessary. It's a dog of a format."
- distributed on CDROM, VMAP0 takes 4 CDs to cover the world
- Although VMAP0 is available (see
Locations - International) is it extremely low detail. VMAP1 should
be available, but only an early version of a few tiles has actually been
released (see Data Openness, and
mapAbility.com VMAP1). Making the full (public domain) VMAP1 actually
available will apparently require a very painful, expensive legal
process involving the FOIA (Freedom Of Information Act), which (as of
2006) nobody has yet done.
- a military vector format, originally developed for radar applications
- defines terrain features, both natural and cultural
- based on 18bit words, requires a particular ordering of features, and
uses feature codes
- reportedly DFAD is not well designed to support 3D visualization
- was DFAD phased out in favor of VPF? yes, and DFAD
is basically unavailable unless you are working for the DOD
- it is/was read by some high-end commercial Vis-Sim software (e.g..
MultiGen, Coryphaeus, EaSIEST)