Geologic Maps vs Soil Maps: Which to Use

For most projects, use a soil map when you need to know about the ground near the surface (depths of 0–2 m) for foundations, drainage, agriculture, or excavation; use a geologic map when you need to understand deeper rock layers, faults, or groundwater aquifers. The two maps answer different questions, and a well-planned project often consults both. This article explains what each map shows, how they are made, and how to choose the right one for your specific task.

Formula

The pace formula is simple: divide the total time by the distance. In symbols:

pace (min/km) = time (min) ÷ distance (km)

Speed is the inverse: speed (km/h) = distance (km) ÷ time (hours). Because time is given in minutes, convert to hours by dividing by 60.

Variables:

• t = total finish time in minutes
• d = distance in kilometers
• pace = average time per kilometer (min/km)
• speed = average speed in kilometers per hour

Worked Example

Using the default values from the calculator: finish time = 50 minutes, distance = 10 km.

Step 1: Calculate pace.
pace = 50 ÷ 10 = 5.0 min/km

Step 2: Calculate speed.
First convert time to hours: 50 min ÷ 60 = 0.8333 hours.
speed = 10 ÷ 0.8333 = 12.0 km/h

The calculator shows an average pace of 5.00 min/km and an average speed of 12.00 km/h.

What a Geologic Map Shows

A geologic map displays the distribution of rock types, sediment, and structural features such as faults, folds, and fractures at the Earth’s surface. It uses colors and symbols to represent formations—bodies of rock with similar age and origin. Geologic maps also include strike-and-dip symbols that tell you the orientation of tilted rock layers, and they often show the location of quarries, mines, and springs.

These maps are created by field geologists who walk the terrain, record rock exposures, and then correlate them with subsurface data from drill holes and geophysical surveys. The result is a two-dimensional representation of the top of the bedrock, plus information about what lies beneath. Many geologic maps also include cross-sections—vertical slices that show how rock layers extend underground.

For a project, a geologic map is essential when you need to know:

• Depth to bedrock (for pile foundations or tunneling)
• Rock type and strength (for excavation or quarrying)
• Fault locations (for seismic hazard assessment)
• Groundwater aquifer boundaries and recharge zones
• Potential for landslides or subsidence in certain rock units

What a Soil Map Shows

A soil map, often called a surficial geology map or pedologic map, classifies the unconsolidated material at the Earth’s surface—the soil and sediment that sits on top of bedrock. It delineates soil series or types based on properties like texture (sand, silt, clay), drainage, color, and depth to a restrictive layer. In the United States, the USDA Natural Resources Conservation Service (NRCS) produces detailed soil surveys that include maps at scales of 1:12,000 to 1:24,000, with accompanying data tables.

Soil maps are built from auger borings, test pits, and analysis of soil profiles. Surveyors dig holes, describe the layers, and sample the soil for laboratory tests. The boundaries between map units are drawn where soil properties change significantly. Unlike geologic maps, soil maps focus on the top few meters—the zone that affects plant roots, septic systems, and shallow foundations.

Use a soil map when you need:

• Bearing capacity for a shallow foundation (footings)
• Permeability and drainage for septic fields or stormwater infiltration
• Erosion potential and slope stability
• Agricultural capability and irrigation needs
• Excavation difficulty in unconsolidated material

Key Differences at a Glance

The two map types differ in scale, content, and purpose. Here is a side-by-side comparison:

• Depth focus: Geologic maps show bedrock and deep structure; soil maps show the top 1–2 meters of loose material.
• Scale: Geologic maps are often regional (1:100,000 or smaller); soil maps are typically more detailed (1:12,000 to 1:24,000).
• Data collection: Geologic maps rely on outcrops and geophysics; soil maps rely on borings and pits.
• Symbols: Geologic maps use formation codes and strike-dip symbols; soil maps use soil series names and slope classes.
• Typical users: Geologic maps are used by engineers, hydrogeologists, and planners; soil maps are used by farmers, landscapers, and civil engineers.

How to Choose for Your Project

Start by asking what part of the ground matters. If your project involves a building foundation, a road, or a pipeline, you need to know both the soil and the rock below. The soil map tells you if the soil can support a footing; the geologic map tells you if bedrock is shallow enough to bear on or if you must drive piles.

For a residential septic system, the soil map is the primary tool because it classifies drainage and percolation rates. A geologic map may be secondary, but it can warn you if the area is underlain by limestone with sinkholes or by clay that expands.

For a groundwater well, the geologic map is more important because it shows aquifer units and their extent. Soil maps rarely reach the water table.

For a large infrastructure project, both are necessary. Geotechnical engineers typically review both maps before planning a site investigation. The maps guide where to drill boreholes and what to expect at depth.

Common Problems and Misconceptions

One common misconception is that a geologic map shows the soil type. It does not—it shows rock and sediment formations, which may include unconsolidated deposits like sand and gravel, but those are classified by geologic origin, not by engineering properties. A sand deposit of glacial origin may behave differently from a river sand, even if both are called “sand” on a soil map.

Another issue is scale. A geologic map at 1:250,000 is too coarse for a single building site. You cannot rely on it to predict conditions at your exact location. Similarly, a soil map may have map units that contain inclusions of other soils, and the actual conditions can vary within a unit.

Both maps are interpretations, not ground truth. They are based on limited observations and extrapolation. Always verify with on-site testing—boreholes, test pits, or geophysical surveys—before final design.

Also note that soil maps are not always available for remote areas, and geologic maps may be outdated. Check the publication date and the mapping scale before using them for engineering decisions.

How to Read and Access These Maps

Geologic maps use a standard legend that explains colors and symbols. The map unit symbols often combine a letter for the geologic period (e.g., “Q” for Quaternary) and a lowercase letter for the rock type (e.g., “al” for alluvium). The legend also includes a description of each unit and its age.

Soil maps use a similar color scheme but the legend lists soil series names, slope ranges, and sometimes a land capability class. In the NRCS Web Soil Survey, you can zoom to your area, draw a polygon, and generate a report with soil properties like available water capacity, saturated hydraulic conductivity, and depth to bedrock.

Both types are available from government agencies. In the US, geologic maps are published by the US Geological Survey (USGS) and state geological surveys. Soil maps are published by the USDA NRCS. Many are available free online as GIS layers or PDFs.

Frequently Asked Questions

Can I use a soil map instead of a geotechnical report?
No. A soil map gives general classifications and properties, but it cannot replace site-specific testing. Geotechnical reports include laboratory tests, bearing capacity calculations, and recommendations. Use the map to plan the investigation, not to skip it.

Do geologic maps show groundwater depth?
No, geologic maps do not show the water table. They show the type of rock and sediment, which can help you infer where water might be, but actual groundwater levels require monitoring wells or piezometers.

What is the difference between a soil map and a surficial geology map?
A soil map classifies the topsoil and subsoil for agricultural and engineering purposes. A surficial geology map shows the distribution of unconsolidated deposits (like glacial till, alluvium, or colluvium) and their origin and age. Surficial geology maps are more like geologic maps but focused on the surface sediments. For many projects, either can be useful, but they emphasize different properties.

How accurate are these maps?
Accuracy depends on scale and the density of field observations. A 1:24,000 soil map is fairly accurate for regional planning but may miss small pockets of different soil. Geologic maps at 1:100,000 have even more generalization. Always treat boundaries as approximate.

Which map should I use for a solar farm?
For a solar farm, you need both. The soil map helps with drainage and foundation design for the panels; the geologic map helps with large-scale grading and potential for rock excavation. If the site has steep slopes, also check a landslide susceptibility map.

Are there free online sources for these maps?
Yes. The USGS National Map provides geologic maps, and the NRCS Web Soil Survey provides soil maps. Many states also have online GIS portals. For international projects, check the national geological survey and agricultural ministry.

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in t | Finish time | min | 50 | 0
in d | Distance | km | 10 | 0
out pace | Average pace | min/km | t / d | 2
out speed | Average speed | km/h | d / (t / 60) | 2