earth science

USGS DEM

USGS DEM is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand USGS DEM rather than just read about it. In short: The USGS DEM standard is a geospatial file format developed by the United States Geological Survey for storing a raster-based digital elevation model. It is an open standard, and is used throughout the world.

Key takeaways

USGS DEM belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect USGS DEM to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of USGS DEM from memory before moving on to harder problems.

Reference excerpt

The USGS DEM standard is a geospatial file format developed by the United States Geological Survey for storing a raster-based digital elevation model. It is an open standard, and is used throughout the world. It has been superseded by the USGS's own SDTS format but the format remains popular due to large numbers of legacy files, self-containment, relatively simple field structure and broad, mature software support.

DEM Level A USGS DEM can be classified into one of four levels of quality. This is due to the multiple methods of data collection, and certainty in the data.

Format Structure The USGS DEM format is a self-contained (single file) set of ASCII-encoded (text) 1024-byte (1024 ASCII chars) blocks that fall into three record categories called A, B, and C. There is no cross-platform ambiguity since line ending control codes are not used, and all data including numbers is represented in readable text form. There is no known binary analogue of the format, although it is common practice to compress the files with gzip. Floating-point numbers are encoded using Fortran scientific notation, so C/C++ programs need to swap the "D" exponent-indicating character with "E" when parsing (and vice versa when writing). A record fields hold the origin, type, summary statistics and the measurement systems used by the profiles. The A record appears once as the file's header, the C record also appears once as the trailer, and multiple B records (called profiles) comprise the elevation data. A and C records each fit within one block, but a single B record typically requires multiple blocks. When such block-spanning occurs, data are shifted to start cleanly on each block boundary. A records also come in "old" and "new" flavors, because the USGS added several fields to the A record. One of the key items is the quadrangle, which is a set of four terrestrial coordinates describing the four-sided polygon enclosing the area of interest.

B records (profiles) are a variable-length longitudinal column of raster elevations that start at a specified location. They are some multiple of 1024 bytes long and contain a small header summarizing the profile. The elevations are contiguous; breaks or other discontinuities are expressed using "void" elevations of value -32767. Each elevation is described as a six-character readable integer occupying a fixed location in a block. The profile header only appears in the first block, so subsequent blocks hold more elevation values. When reading the DEM file from first byte to last, one reads the profiles as columns from west to east. The elevations within a profile run from south to north. The variable-location and variable-length nature of profiles stems mainly from the use of the UTM (Universal Transverse Mercator) ground reference system. Since measurements within UTM employ fixed distances (e.g., 30 meters between elevation samples), the quadrangle must slightly distort to map such locations onto the spherical Earth. This distortion usually manifests as a rotated square, hence the elevation columns near the east and west edges start more northward and contain fewer samples. C records contain root-mean squared error (RMSE) quality control data, using ten six-character integer fields.

External links Standards for digital elevation models, U.S. Department of the Interior, U.S. Geological Survey, National Mapping Division (1992): Part 1: General (1992) Part 2: Specifications (1995) Part 3: Quality control (1992) Errata and changes (1998) Sources for USGS DEMs: Canadian Digital Elevation Data, Level 1

Worked examples

Example 1 — a first encounter with USGS DEM

Start with the simplest possible case. Write down what USGS DEM claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to USGS DEM before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about USGS DEM ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of USGS DEM

In research: USGS DEM appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses USGS DEM in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: USGS DEM is common in secondary-school and first-year university syllabi. It links to neighbouring topics Digital elevation models, GIS raster file formats, United States Geological Survey, so understanding it makes those chapters shorter.
In everyday life: Look for USGS DEM outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study USGS DEM in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what USGS DEM means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain USGS DEM out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is USGS DEM in simple terms?

Why does USGS DEM matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study USGS DEM?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on USGS DEM.