Digital Elevation Model (DEM)Digital Elevation Model (DEM)Data SourcesData Sources

That You Can Get EasilyThat You Can Get Easily

Quadrilateral Data Structure

Geodetic latitudeand longitude onWGS 84 ellipsoid(NAD 83)

Grid cell resolutionconstant N-S butprogressively finer E-W resolution fromequator to pole

ETOPO2 Global DEMfrom National Geophysical Data Center

Worldwide Land And Worldwide Land And Sea Elevation DataSea Elevation Data

In MetersIn Meters(3.7x3.7km cells at equator)(3.7x3.7km cells at equator)

Technical Specifications(0,0) (10799,0)

(0,5399) (10799,5399)

2-byte signed integer data5400 rows by 10800

columns116,640,000 byte dataset

Row Major Order

CleanTOPO2

Global Land One-km Base Elevation (GLOBE) DEM data from NGDC

•30” x 30” grid cells for the earth30” x 30” grid cells for the earth•Metric data for land areas, NoDataMetric data for land areas, NoData value (-500) given to sea areasvalue (-500) given to sea areas

Spatial Resolution

0.93 x 0.93 km0.93 x 0.93 kmCells at equatorCells at equator

Globe 30” x 30” grid cell data structure and tiling, format, and dataset size

USGS GTOPO30 Global Land DEM

•Integer Metric data for land areas, Integer Metric data for land areas, NoData value (-9999) given to NoData value (-9999) given to sea areas.sea areas.•30” x 30” grid cells for the earth.30” x 30” grid cells for the earth.•Single GRID dataset on ESRISingle GRID dataset on ESRI Data & Maps (21,600 rows x Data & Maps (21,600 rows x 43,200 cols. [1.866GB file])43,200 cols. [1.866GB file])

USGS Version Has 33 Tiles

Shuttle Radar Topography Mission (SRTM)3 arc-second (90m) data from USGS EROS

Grid 2

Each grid is divided into1 degree tiles (1200 x 1200)

for land areas

•Data are First Data are First Reflective ReturnsReflective Returns•Data Completeness Data Completeness ProblemsProblems

Voids areVoids areFilled in ESRIFilled in ESRI

Maps and DataMaps and Data90m SRTM Dataset90m SRTM Dataset

Data areData areCompressedCompressed

From 34GB toFrom 34GB to4.33GB4.33GB

Shuttle Radar Topography Mission (SRTM)1 arc-second (30m) data from USGS EROS

Each grid is divided into1 degree tiles (1200 x 1200)

for land areas

Grid 4

SRTM Data are also availablefrom The National Map Seamless Server

National Elevation Dataset (NED)from The National Map Seamless Server

National Elevation Dataset (NED)Data Structure

•One-second grid cells for all One-second grid cells for all U.S. states except AlaskaU.S. states except Alaska•31m N-S resolution31m N-S resolution•28m to 20m E-W resolution28m to 20m E-W resolution•NAD 83 lat./long. cell centersNAD 83 lat./long. cell centers from 25from 25° to ° to 50°50° N•Full dataset is about 60GBFull dataset is about 60GB

Two-second Grid For Alaska

•NAD 83 lat./long. cell centersNAD 83 lat./long. cell centers•62m N-S resolution62m N-S resolution•38m to 21m E-W resolution38m to 21m E-W resolution from 52° to 70°from 52° to 70° N

National Elevation Dataset (NED)Data Format and Tiling Scheme

National Elevation Dataset (NED)

1/31/3rdrd (10m) and 1/9 (10m) and 1/9thth (3.3m) (3.3m) Arc-second Grids for Arc-second Grids for

Certain Areas from Certain Areas from The The National MapNational Map Seamless Seamless

ServerServer

You can buy theYou can buy theData for each stateData for each state

In ArcGIS GRIDIn ArcGIS GRIDformatformat

Light Detection And Ranging(LIDAR) Data

Uses a laser and sensor to transmit and receive pulses

from reflected surfaces

Post processing of the return data produces ahighly accurate DEM,typically with 1 meter

spatial resolution.

LIDAR DEMsavailable only forsmall local areas

USGSUSGSNational Land Cover DataNational Land Cover Data

(NLCD) 2001(NLCD) 20011”x1” Grid1”x1” Grid

From The National Map Seamless Server