Open HGT File Online Free (No Software Required)

HGT files serve as the backbone for high-precision terrain modeling, originating from NASA’s Shuttle Radar Topography Mission (SRTM). These files store elevation data in a compact, binary grid format, representing specific geographical "tiles" of the Earth's surface. Because they lack standard image headers, they require specialized handling to translate raw binary integers into visual or functional landscapes.

Real-World Use Cases

Civil Engineering and Site Analysis

Engineers utilize HGT data during the preliminary stages of infrastructure planning. By importing these files into CAD or BIM software, they can calculate slope gradients, identify potential flood zones, and determine optimal drainage routes without deploying manual survey teams to the field. This is particularly vital for telecommunications companies plotting line-of-sight for 5G towers across undulating terrain.

Flight Simulation and Aerospace Development

The gaming and aviation industries rely on HGT files to generate realistic, three-dimensional earth environments. Flight simulator developers parse this elevation data to ensure that mountain ranges and valley floors match real-world coordinates precisely. This ensures that pilots training in virtual cockpits experience a 1:1 representation of the physical world’s topography.

Hydrological Research and Conservation

Environmental scientists use SRTM data to map watersheds and predict the movement of pollutants or runoff. By analyzing a series of HGT tiles, researchers can model how water accumulates in specific basins. This data informs conservation efforts and helps local governments create accurate disaster preparedness maps for mountainous regions prone to landslides.

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Step-by-Step Guide

1. Verify Coordinate Alignment: Ensure the filename matches the global coordinate system (e.g., N35W090.hgt). The filename itself acts as the primary metadata, indicating the southwestern corner of the 1x1 degree tile.
2. Select a Compatible Interpreter: Use a tool capable of reading raw 16-bit signed integers. Since HGT files do not contain internal metadata regarding their dimensions, you must manually select the grid resolution (typically 1201x1201 or 3601x3601 pixels).
3. Handle Big-Endian Conversion: Most modern processors are little-endian, but HGT files are stored in big-endian format. Ensure your software or conversion tool swaps the byte order during the import process to prevent "noise" or corrupted elevation readings.
4. Identify Voids and Data Gaps: Scan the file for values of -32768. This specific integer represents a "void" where radar shadows or water bodies prevented a clean reading. Use an interpolation algorithm to fill these gaps for a seamless mesh.
5. Apply Georeferencing: Once the raw binary is interpreted, assign a spatial reference system, typically WGS84. This allows the elevation data to overlay perfectly with satellite imagery or vector maps.
6. Export to Accessible Formats: Convert the refined HGT data into a GeoTIFF or STL file. This makes the terrain data compatible with standard 3D printers, game engines, or traditional GIS software.

Technical Details

The HGT format is a raw binary stream of 16-bit signed integers. Unlike modern image formats, it contains no headers, footers, or compression signatures. The simplicity of the structure—a continuous sequence of elevation values—is exactly what makes it difficult for standard software to recognize without prior parameters.

Structure and Resolution

HGT files are provided in two primary resolutions: SRTM-3 (3 arc-seconds) and SRTM-1 (1 arc-second). An SRTM-3 file contains a grid of 1201 x 1201 samples, totaling approximately 2.8 MB. An SRTM-1 file consists of a 3601 x 3601 grid, resulting in a file size of roughly 24.7 MB. Each sample represents the elevation in meters above the WGS84 ellipsoid.

Encoding and Compression

There is no internal compression within an HGT file. Every two bytes represent a single elevation point. The data is written row by row, starting from the northern latitude down to the southern latitude. Because it uses signed 16-bit integers, it can represent elevations ranging from -32,767 to 32,767 meters, though the extreme negative value (-32,768) is strictly reserved as a flag for missing data.

FAQ

Why does my HGT file look like a distorted black and white mess in an image viewer?

Most image viewers attempt to read HGT files as 8-bit grayscale images, but the data is actually 16-bit big-endian binary. Because the byte order is reversed compared to standard Windows or Linux protocols, the software perceives the elevation data as random noise. You must use a specialized converter to reorder the bytes and map the 16-bit values to a visible range.

Can I merge multiple HGT files into a single terrain map?

Yes, but you must "stitch" them based on their filenames, which indicate their geographic location. Since each file represents a 1x1 degree square, they can be tiled horizontally and vertically. Professional GIS tools or robust online converters can perform this mosaicking process to create a continuous Digital Elevation Model (DEM) over a large region.

What is the difference between SRTM-1 and SRTM-3 HGT files?

The difference lies in the sampling density or "pixel" size of the terrain. SRTM-1 provides a data point every 30 meters approximately, offering high-detail terrain useful for localized engineering. SRTM-3 provides a point every 90 meters, which is more efficient for large-scale geographic modeling where extreme precision is less critical than processing speed.

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