2.3 Desktop Inventory, GIS Data & Field Reconnaissance

A thorough site inventory is the foundational prerequisite for sustainable land planning, site engineering, and ecological design. In landscape architectural practice, candidates must distinguish between two fundamental terms that CLARB rigorously tests:

• Site Inventory: The objective, factual collection, mapping, and cataloging of all existing physical, biological, and cultural conditions on and surrounding a property parcel without evaluating their design suitability.
• Site Analysis: The qualitative and quantitative evaluation of the inventory data to determine how existing site conditions present developmental opportunities and constraints relative to the proposed program.

Attempting site analysis or schematic layout without a rigorous baseline inventory invariably results in unforeseen subsurface complications, regulatory permitting delays, budget overruns, and severe public opposition.

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Desktop Inventory vs. Field Reconnaissance

A professional site investigation follows a deliberate two-stage workflow: an initial desktop inventory followed by targeted on-site field reconnaissance.

Two-Stage Site Investigation Workflow
+-------------------------------------------------------------------------+
|                        STAGE 1: DESKTOP INVENTORY                       |
|  • Public GIS Data (Parcels, Zoning, Utilities)                         |
|  • USDA NRCS Soil Surveys (SSURGO: Hydric soils, depth to bedrock)      |
|  • FEMA Flood Maps (NFHL: 100-year floodplains, floodways)             |
|  • USGS 3DEP Elevation & Hydrology (NHD)                                |
|  • NOAA Atlas 14 Precipitation Data                                     |
|  • Historical Aerial Orthoimagery (Historic land uses / Brownfields)    |
+-------------------------------------------------------------------------+
                                     │
                                     ▼ (Generates Preliminary Base Map & Checklists)
+-------------------------------------------------------------------------+
|                     STAGE 2: FIELD RECONNAISSANCE                       |
|  • Ground-Truthing Desktop Data (Confirming wetlands, tree health)      |
|  • Empirical Primary Measurements (Test pits, percolation, GNSS points) |
|  • Sensory Mapping (Viewshed quality, microclimates, noxious odors)     |
|  • Infrastructure Verification (Manhole inverts, overhead wires)       |
|  • Geotagged Photographic Documentation & Site Logs                     |
+-------------------------------------------------------------------------+

Primary vs. Secondary Data Sources

• Primary Data: Information collected firsthand through direct empirical measurement, field observation, or original testing on the site. Examples include geotechnical soil test borings, percolation rates, topographic total station survey shots, health evaluations of specimen trees, traffic volume counts, and pedestrian intercept interviews.
• Secondary Data: Pre-existing spatial, statistical, or environmental records compiled by government agencies, commercial vendors, or academic researchers for other purposes. Examples include USDA Web Soil Survey reports, USGS topographic quadrangle sheets, county tax assessor parcel maps, and municipal utility atlas drawings.

Exam Trap: Secondary data provides invaluable regional context, but it must never be used as the sole basis for construction documentation. For instance, the USDA Web Soil Survey was created at a broad landscape scale (1:12,000 to 1:24,000) for agricultural and regional planning. It cannot replace a site-specific geotechnical boring report when designing building foundations, retaining walls, or stormwater bioretention basins.

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GIS Spatial Data Layers & Authoritative Public Repositories

Geographic Information Systems (GIS) synthesize multivariate spatial data layers into an interactive digital environment. Spatial data operates in two fundamental formats:

1. Vector Data: Represents discrete spatial entities using geometry:
   • Points: Specific coordinates (e.g., specimen trees, utility manholes, soil boring locations).
   • Lines / Polylines: Linear features (e.g., stream centerlines, property boundaries, contour lines, water mains).
   • Polygons: Enclosed areas (e.g., zoning districts, parcel boundaries, wetland limits, building footprints).
2. Raster Data: Represents continuous surfaces using a regular grid of pixel cells, where each cell contains a specific attribute value. Examples include aerial orthophotographs, digital satellite imagery, Digital Elevation Models (DEMs), and derived slope or solar radiation surfaces.

Authoritative Federal and Regional Data Sources

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Base Map Preparation & Field Reconnaissance Protocols

The base map is the master 2D or 3D digital drawing upon which all subsequent inventory mappings, site analyses, schematic concepts, and construction documents are overlaid. Preparing an accurate base map requires harmonizing disparate datasets into a uniform, verified coordinate environment.

Base Map Assembly Steps

1. Coordinate Harmonization: Project all spatial files into the project's designated State Plane Coordinate System (SPCS) and horizontal/vertical datum (e.g., NAD83, NAVD88).
2. Boundary Anchoring: Import the certified boundary survey from the licensed Professional Land Surveyor as the primary legal constraint layer.
3. Topography & Features: Overlay existing contours, breaklines, spot elevations, structures, utility easements, and mature trees (> 6 inches DBH).
4. Contextual Buffers: Include a surrounding contextual buffer extending at least 100 to 300 feet beyond parcel boundaries to capture off-site influences: adjacent building heights, driveways, street intersections, downstream drainage paths, and neighboring mature tree canopies.
5. Drawing Standardization: Establish a bold graphic scale, clear north arrow (differentiating true north, grid north, and magnetic north), sheet index, and title block containing revision dates.

Field Data Gathering Techniques

During on-site field visits, landscape architects verify desktop assumptions using specialized field instruments:

• High-Accuracy GNSS Field Data Collectors: Handheld field computers equipped with GPS/GLONASS receivers using real-time SBAS or cellular RTK correction to capture field assets (e.g., unrecorded storm outfalls, invasive plant patches, bench locations) with sub-meter to centimeter precision.
• Optical and Laser Tools: Handheld laser rangefinders (for measuring setbacks and building heights), Abney hand levels or clinometers (for verifying localized slope gradients), and diameter tapes (for measuring tree Diameter at Breast Height / DBH at 4.5 feet above grade).
• Subsurface Probing: Hand augers and soil probes to quickly inspect topsoil depth, soil texture, compaction layers, and seasonal soil mottling (redoximorphic features indicating seasonal high water tables).
• Systematic Photographic Documentation: Photographs must be keyed to a master site map indicating camera position and direction of view (cardinal direction). Photographers should capture wide-angle contextual panoramas paired with close-up macro shots of structural cracking, erosion rills, tree root defects, or drainage outfalls. Geotagged smartphone or digital camera images preserve location metadata in EXIF files.

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