LDART Help
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Spatial Boundaries | Selected Dates | Projection | Datums | QA Level and Sensor Type | Your E-mail Address | Bin Method | File Format | The Start Button
Entering Your Spatial Boundaries
LDART will extract LIDAR data based on the boundaries that you see in the four text fields North, South, East, and West. These four fields, which represent the northern, southern, eastern, and western edges of your data request, can be modified here to narrow a particular area of interest. These fields should have been defined using the mapping application on the previous page. If they weren't, you may either use the Back button on your browser to return to that application to reselect your geographic boundaries, or type them directly. It is important to remember that LDART requires that longitudes be negative west, meaning that longitudes west of the prime meridian (all of the United States) are all negative. This means that -80 is further west than -79. If you reverse your longitudes or latitudes, LDART will let you know when you click the Start button. It is also important to remember that LIDAR data are very dense, and large spatial boundaries can result in immense data files. LDART has several warnings in place to remind you of this.
Selecting Your Desired Dates
Click the "Edit Dates" button to bring up the following dialog.
LDART will select LIDAR data based on the dates that you see in the list box labeled Selected Date(s):. These dates should have been selected by using the mapping application on the previous page. If they weren't, you may either use the Back button on your browser to return to that application to reselect your dates, or edit them by clicking the Edit Dates button and using the Edit Dates dialog. Using this dialog, you can select dates from the Available Dates: list box by clicking the Add button. Dates can be removed by selecting them from the Selected Date(s): list box and clicking the Remove button, or by clicking the Remove All button.
Selecting Your Projection Parameters
Projection - Geographic (Lat/Lon)
LDART will return your data in a geographic projection, which is to say, no projection at all. The horizontal values will be returned as decimal degrees in latitude and longitude. Note: choosing Geographic (Lat/Lon) from the Projection drop-down list will change the Projection Units drop-down list to contain only Decimal Degrees. You should hear a "ping" as this happens.
Projection - UTM
LDART will return your data in a Universal Transverse Mercator (UTM) projection for any UTM zone in the contiguous 48 states (chosen from the Zone drop-down list). The horizontal values will be returned as meters or feet, as chosen from the Projection Units drop-down list.
Projection - State Plane 1983
LDART will return your data in a State Plane 1983 projection for any State Plane zone in the contiguous 48 states (chosen from the Zone drop-down list). The horizontal values will be returned as meters or feet, as chosen from the Projection Units drop-down list. Note: choosing State Plane 1983 from the Projection drop-down list will change your Horizontal Datum to NAD83. You should hear a "ping" as this happens.
Projection - State Plane 1927
LDART will return your data in a State Plane 1927 projection for any State Plane zone in the contiguous 48 states (chosen from the Zone drop-down list). The horizontal values will be returned as meters or feet, as chosen from the Projection Units drop-down list. Note: choosing State Plane 1927 from the Projection drop-down list will change your Horizontal Datum and Vertical Datum to NAD27 and NGVD29, respectively. You should hear a "ping" as this happens.
Selecting Your Vertical And Horizontal Datums
Horizontal Datum - ITRF-94; Vertical Datum - WGS84
LDART will return your x, y, and z (elevation) coordinates referenced to the WGS84 ellipsoid. This is not a "sea-level" reference system, which means that an elevation value of 0 is most likely not anywhere near the mean sea-level value for that area. It is the surface of a mathematically constructed ellipsoid model of the earth. This option will be most useful to you if you are concerned with getting this LIDAR data in the most accurate form, uncorrupted by any datum changes, which can introduce error. Note: choosing ITRF-94 from the Horizontal Datum drop-down list will restrict the Vertical Datum drop-down list to Ellipsoid. You should hear a "ping" as this happens.
Horizontal Datum - NAD83; Vertical Datum - NAD83
LDART will return your x, y, and z (elevation) coordinates referenced to the NAD83 (GRS-80) ellipsoid. This is not a "sea-level" reference system, which means that an elevation value of 0 is most likely not anywhere near the mean-sea level value for that area. It is the surface of a mathematically constructed ellipsoid model of the earth.
Horizontal Datum - NAD83; Vertical Datum - NAVD88
LDART will return your x and y coordinates referenced to the NAD83 datum. Your z coordinates (elevation) will be referenced to the North American Vertical Datum of 1988 (NAVD88). This is a "sea-level" reference system, which means that an elevation value of 0 is somewhere near the mean sea-level value for that area. Of the two "mean sea-level" options, NAVD88 is preferred over NGVD29. If this is your only elevation data set, or if your other elevation data are referenced to NAVD88, then this is the option you should select.
Horizontal Datum - NAD83; Vertical Datum - NGVD29
LDART will return your x and y coordinates referenced to the NAD83 datum. Your z coordinates (elevation) will be referenced to the North Geodetic Vertical Datum of 1929 (NGVD29). This is a "sea-level" reference system, which means that an elevation value of 0 is somewhere near the mean sea-level value for that area. Of the two "mean sea-level" options, NAVD88 is preferred over NGVD29. If this is your only elevation data set, then you should probably choose NAVD88. If your other elevation data sets are referenced to NGVD29, then this is the option you should select.
Horizontal Datum - NAD27; Vertical Datum - NGVD29
LDART will return your x and y coordinates referenced to the NAD27 datum. Your z coordinates (elevation) will be referenced to the North Geodetic Vertical Datum of 1929 (NGVD29). This is a "sea-level" reference system, which means that an elevation value of 0 is somewhere near the mean sea-level value for that area. Of the two "mean sea-level" options, NAVD88 (with NAD83 for the horizontal) is preferred over NGVD29. If this is your only elevation data set, then you should probably choose NAVD88. If your other elevation data sets are referenced to NGVD29, then this is the option you should select. Note: choosing NAD27 from the Horizontal datum drop-down list will restrict the Vertical Datum drop-down list to NGVD29. You should hear a "ping" as this happens.
Selecting Your Bin Method
- Choosing None from the Bin Method drop-down list will create a text file containing all of the LIDAR data points collected on the date(s) and in the area you specify. This will also set the File Format drop-down list to x,y,z points. You should hear a "ping" as this happens. This raw data will be unaltered by any sort of binning process, but will also be more difficult to work with because it is not image data. This option is most useful for investigators who want to work closely with the raw data and find new uses for them.
- Selecting Smoothed from the Bin Method drop-down list will form a regular grid with the bounds and cell size you entered. All cells in that grid will be filled using an Inverse Distance Weighting (IDW) method. If fewer than 6 values are found inside a 10-meter radius, the cell will be filled with a no-data code, -999999 in the case of a binary raster file, or 255 in the case of a Geotiff. If File Format is x,y,z points, then these cells will not be assigned any value. Note: choosing this binning method will check the Filled check box. You should hear a "ping" as this happens.
- Selecting Average from the Bin Method drop-down list will form a regular grid with the bounds and cell size you entered. The values for a given cell in that grid will be determined by averaging all of the values in that cell. If no values are found inside the cell, it will be filled with a no-data code, -999999 in the case of a binary raster file, or 255 in the case of a Geotiff. If File Format is x,y,z points, then these cells will not be assigned any value. Averaging could be useful for calculating beach volume.
- Selecting Minimum from the Bin Method drop-down list will form a regular grid with the bounds and cell size you entered. The values for a given cell in that grid will be determined by taking the minimum value of all the values in that cell. If no values are found inside the cell, it will be filled with a no-data code, -999999 in the case of a binary raster file, or 255 in the case of a Geotiff. If File Format is x,y,z points, then these cells will not be assigned any value. This option could be useful for finding the ground elevations in a heavily vegetated area.
- Selecting Maximum from the Bin Method drop-down list will form a regular grid with the bounds and cell size you entered. The values for a given cell in that grid will be determined by taking the maximum value of all the values in that cell. If no values are found inside the cell, it will be filled with a no-data code (usually -999999) in the case of a File Format of either binary raster file or Geotiff. If File Format is x,y,z points, then these cells will not be assigned any value. This option could be useful for finding the canopy elevations in a heavily vegetated area.
- Checking the Filled check box will form a regular grid with the bounds and cell size you entered. It will use the binning method of your choice (chosen from the Bin Method drop-down list) to create a raster file in the file format of your choice (chosen from the File Format drop-down list) With the Filled check box checked, some cell values that would otherwise have been filled with a "no-data" code will now be filled by interpolating from cells with data values already in them. Filled is checked by default.
Selecting Your File Format
- Selecting x,y,z points - see help section for selecting None from the Bin Method drop-down list.
- Selecting binary raster file will form a regular grid with the bounds and cell size you entered. It will use the binning method of your choice (chosen from the Bin Method drop-down list) to create a binary raster file. This format is described in the e-mail delivered by LDART and also in the help for ArcView®.
- Selecting GeoTiff will form a regular grid with the bounds and cell size you entered. It will use the binning method of your choice (chosen from the Bin Method drop-down list) to create an 8-bit raster file in the GeoTiff format. For compatibility with ArcView®, a TIFF world file (.tfw) containing georeferencing information will be included with the image file. However, if you have a software package that can read GeoTiffs, this TIFF world file is not necessary. In addition, this GeoTiff can be viewed in most standard image-viewing packages that read the TIFF format, since the extra georeferencing information in the GeoTiff file is usually ignored. It can also be inserted into Microsoft® Word® and viewed that way. If you are interested in serious data analysis, this file format is not recommended, as the elevation values are not saved in the image file. The image is colored according to elevation, and you can estimate the elevation values in the image by looking at the color bar embedded in the upper right-hand side, but this is not a recommended method. Instead, download the data in the "binary raster file" format.
- Selecting Contour shapefile will form a regular
grid with the bounds and cell size you entered. It will
then use the binning method of your choice (chosen from
the Bin Method drop-down list), the
grdcontour routine from the Generic
Mapping Tools, and the Center's own software to create
a vector contour in the ESRI® shapefile format. This
format is described in the e-mail delivered by LDART and
also in the help for ArcView.
If file size is an issue, an important consideration when defining the contour interval is the elevation range and variability of the area of interest. Topography with a large range and variability in elevation will generate a large file. To minimize the file size, a larger contour interval or smaller spatial area should be selected.
- Selecting Contour DXF will form a regular
grid with the bounds and cell size you entered. It will
then use the binning method of your choice (chosen from
the Bin Method drop-down list), the
grdcontour routine from the Generic
Mapping Tools, and the Center's own software to create
a vector contour in the AutoCAD® ASCII drawing interchange file
(DXF) format.
If file size is an issue, an important consideration when defining the contour interval is the elevation range and variability of the area of interest. Topography with a large range and variability in elevation will generate a large file. To minimize the file size, a larger contour interval or smaller spatial area should be selected.
- Selecting Ascii Grid File will form a regular grid with the bounds and cell size you entered. It will then use the binning method of your choice (chosen from the Bin Method drop-down list) to create an Ascii Grid File which is readily importable into the Vertical Mapper® module for MapInfo®. See Vertical Mapper User Guide or the online help within Vertical Mapper for details on this format.
- Checking Get Visible Image(s) will create 8-bit gray-scale GeoTiffs in addition to the selected elevation data product. These images will be made by merging all passes from a particular day of flights into a single GeoTiff. One image will be generated for every day of flights. The quality of these images will vary.
Selecting Your "Which LIDAR Return?" Option
LDART currently only contains first return data sets, but we are anticipating adding data sets with multiple returns. If a selected mission has multiple returns, this selection box will be enabled for defining which return to use.
Selecting Your Contour Option
If either the contour shapefile or contour DXF file option is selected from the File Format selection box, the Contour Option will be enabled. The contour option, Interval, will extract contours at the interval entered in the textbox field. The contour option, Single Elevation, will extract a contour only at the elevation value entered in the textbox field.
Your E-mail Address
Please enter a valid email address. LDART will send an email to the entered address on completion of procesisng. The email indicates your data request has been processed and provides a link for downloading the data set.
The Start Button
The Start button should be green by default; when it is clicked, it should turn red and will then be labeled Wait. While you are waiting, LDART is taking the parameters you have entered or selected and is counting how many data points your request will generate (or how big your raster file will be). When it is finished, a message window will pop up with a report of the number of points and/or the potential file size, and ask you if you would like to continue. Clicking Continue will send your request to the server. When your data file has been created, LDART will send you an e-mail with instructions on how to retrieve your file, along with a complete metadata record. Clicking Cancel will cancel your request and allow you to edit your parameters.