Interpolation — As used in this manual, interpolation is the estimation of z-values at a point with x/y coordinates, based on the known z-values of surrounding points. There are many different forms of interpolation: ANUDEM — A variation of spline interpolation optimized for the creation of hydrologically correct (hydro-enforced) terrain models. Inverse Distance Weighted Interpolation — A method for interpolating the elevations of new points by using a linearly weighted combination of values from nearby points, where the weight is a function of inverse distance. Kriging — A method for interpolating the elevations of new points by using geostatistical techniques that weight the surrounding measured values to derive a prediction for each location. However, the weights are based not only on the distance between the measured points and the prediction location but also on the strength of the overall correlation among the measured points. With kriging, the data are used to define the spatial correlation model to determine the weights for neighboring samples that are used to fit a surface to the points. Natural Neighbor Interpolation — A method for interpolating the elevations of new points by using an area based weighting scheme to interpolate heights. Spline — A method for interpolating the elevations of new points by using a mathematical formula to create a surface that minimizes the overall surface curvatures, resulting in a smooth surface that passes through the input points. Intervisibility — The identification of all points that are visible from a designated input point. Generally synonymous with “viewshed” and “line-of-sight.” J K Kalman Filter — A recursive minimum variance estimation technique that uses statistical models to weight each new measurement relative to past information and estimate quantities of interest having statistical characteristics. It can be thought of as a recursive least squares adjustment. Kriging — See Interpolation. L L1 ― 1 frequency of 1575.42 MHz, one of the radio frequencies transmitted by GPS satellites. This frequency carries the C/A code, P-code, and the navigation message. L2 ― Level 2 frequency of 1227.6 MHz, one of the radio frequencies transmitted by GPS satellites. This frequency carries only the P-code.
Vertical Point Spacing ― The vertical distance between laser points from consecutive scan lines when the lidar sensor is tilted into a forward-looking position and laser points “walk up” the face of vertical objects. Viewshed — See Intervisibility Void — Portions of a digital elevation dataset where no elevation data are available. In USGS DEMs, each elevation post located within a void area is assigned a discrete false value representing the void. Treatment of void areas should be documented in the metadata file. Voxel — A three-dimensional "cube" that serves as a counterpart to a traditional pixel, used for three-dimensional visualization of elevation data. A 3-D pixel. W Water — A synonym for tide, in the sense of the elevation of the surface of the hydrosphere at a particular longitude and latitude. High Water — The greatest elevation reached by a rising tide Higher High — The higher of the two high waters occurring on a particular tidal day Higher Low Water — The higher of the two low waters occurring on a particular tidal day. Low — The least elevation reached by a falling tide. Lower Low Water — The lower of two low waters of any tidal day. Lower High Water — The lower of two high waters of any tidal day. Mean High Water (MHW) — The average elevation of all high waters recorded at a particular point or station over a considerable period of time, usually 19 years. Mean Higher High Water (MHHW) — The arithmetic average of the elevations of the higher high waters of a mixed tide over a specific 19-year period Mean Low (MLW) — The average elevation of all low waters recorded at a particular point or station over a considerable period of time, usually 19 years. Mean Lower Low Water (MLLW) — The average elevation of all the lower low waters recorded over a 19-year period. Mean Water Level — The elevation determined by averaging the elevations of the surface of water at equal intervals of time (usually hourly) over a considerable period of time. Waveform Lidar — See Full Waveform Lidar. Weight — A factor by which a quantity is multiplied to increase or decrease the effect of that quantity on the results of an adjustment.