Some elements that may be part of a map are:
- Series name: Name of the area, locality, state
- Map Scale: Shown both graphically and as a representative fraction
- Sheet name, number, edition
- Index to any adjoining sheets
- index to data source
- Meridian arrow
- Contour interval
- Vertical datum
- Horizontal datum
- Legend date of publication
- Name of property or project
- Name of Surveyor/Company
- Border (mandated by statute law in some states)
Six elements comprise what makes good map design
These elements often work together, but can sometimes conflict. The goal of a good map is to balance each of these elements, When conflict does occur, emphasize only those that are most important to the goal of the map.
Clarity - Clarity is the ability of the map to convey its message
- Clarity is achieved by fully examining the objectives of the map, and emphasizing the important points of the map
- Avoid overloading the map with too many details that do not contribute to the overall objectives of the map
- When considerable detail must be included on a map consider using tables to list textual-type data
- Textual elements should take precedence over line work.
- Lines should be broken where text crosses
- Avoid breaking lines in areas involving multiple bends, i.e., use straight portions of lines to place text
Order - Order is the logic of the map; the path that the eye follows when viewing a map
- Avoid areas on the map that are cluttered and confusing
- Avoid making auxiliary map elements (graphical scales, meridian arrows, legends, title blocks, etc.) that are too large, or place to much emphasis on themselves. Remember these auxiliary elements are not the main subjects of the map
- Thru the use of line weights, colors, and font attributes, draw attention to the important elements of the map
- e.g. - title of map, index contours, etc.
- Except in extreme situations, always align the edges of the map with the cardinal directions of the map
Balance - refers to visual balance
- The visual center of the map is slightly above the physical center of the map
- Try to center the subject of the map on the visual center of the map
- Align the longer dimension of the subject area with the longer dimension of the map sheet
- Balance can be achieved thru the use of line weights, fonts, colors, etc. For example,
- Map elements with thicker lines will appear heavier
- Bold text makes elements feel heavier
- Red text appears heavier than yellow
Contrast - Contrast refers to the use of line weights, font characteristics, and colors
- Contrast can be used to enhance balance order and clarity
- E.g. - Use larger, bold font for title of map
- Use yellow, thin lines to de-emphasize contours where they are required for legal purposes, but are not the main purpose of the map. For instance, a presentation to a zone board that is more interested in the esthetics of the land development, but not the specifics.
- Generally, the index contours should be drawn with heavier lines, and "heavier" colors than the intermediate contour lines
Unity - Unity is the interrelationship between backgrounds, shading, font characteristics, and colors on a map
- Be aware that colors are subjective and inter-related. For example, yellow text on white background will de-emphasize the text whereas yellow text on black background will emphasize of the text.
Harmony - is the interrelationship between elements on a map. A good map has harmony between the elements.
- Common errors include
- use of too many font styles
- use of too much color
- meridian arrows that are too large in comparison to other elements
- graphical scales that are too large in comparison to other elements
Judging the balance of these design elements is best achieved by standing back from the map and viewing it in its entirety. Too often, we restrict our view of a map to the particular part of the map we are working on and not the overall product!
The basic questions to answer before drawing a map. REMEMBER: MAP CREATION IS SUBJECTIVE!
- What is the purpose of the map?
- All maps should have a purpose; what they intend to communicate
- Elements that support this purpose should be emphasized.
- Typically, several objectives of the map of the map will conflict. Find the best balance when this happens.
- Who is the intended audience for the map?
- Develop the map to communicate to the intended audience
- Title block
- No standard location for title block, or format for title block.
- Companies often have standard title format and location for clarity and identification of company
- Generally, title of map is emphasized with larger, bold font face
- Normally title of map is written in all capital letters
- Key to a map
- Locate legend so that it creates a good visual balance. If legend is too large, consider placing legend on a separate sheet.
- Symbols should be exact duplicates of that used on map, but can and often are reduced in size.
- Copy the map symbol in CAD and scale to reduce size if appropriate
- Any symbol that is not self-explanatory on the map should appear in the legend.
- All maps should have a graphical scale that will be retained during a copyng process.
- Graphical scales should be narrow in width, at most 0.08 inches in width, or even narrower. Making lines to wide, will result in too much emphasis being place on the scale.
- An accompany verbal scale or representative fraction should be place in the title block
- Should match map sheet dimensions
- Insets, source, date, projection name, explanatory text
- Inset maps often help locate the subject area in a region
- A border frames a map
- Sometimes required by statute laws
- Neat lines can be added inside the border if graticules or grid ticks are used
- Subect area - normally the most important item on the map, and thus should be the most predominant element of the map.
Problem: A subject area with dimensions of 684 ft by 897 ft is to be placed on a map sheet of 11 in. by 17 in. with a 1 in. border. What is the maximum scale that the subject area can be plotted?
- Remove 2 inches from each dimension of the map sheet to account for the border
- Maximum width = 11 - 2 = 9 inches; maximum length = 17 - 2 = 15 inches
- Determine scale by matching longer and shorter dimensions
- In longer direction: 15 in./897 ft = 1/59.8, or closest matching engineer's scale is 1 in. = 60 ft.
- In shorter direction: 9 in/684 ft = 1/76, or closest matching engineer's is 1 in. = 100 ft.
- Use smaller of two scales, so best matching map scale is 1 in. = 100 ft.
- To place border
- Assume that the "press wheels" of the plotter take up 0.5 inches of the map sheet, and that the plotter misses the lower edge by 0.75". These areas will not be part of the active mapping area. They must be taken into account when plotting.
- Length of border is 900 ft (9 in. × 100 ft/in) wide and 1500 ft (15 in. × 100 ft/in.) long in ground units.
- Plot in AutoCAD as
- Set the limits of the drawing to account for the 1 inch border.
- Offset border when plotting to create 1 inch border, e.g. (-0.5, -0.75), and draw the border using the limits command
- Practice this on a blank drawing to that you do not need to wait for a full drawing plot. Check the drawing before send the drawing to a plotter
- Retain the precise shifts with this border, and create a script file
- A script file is an ASCII text file that contains specific CAD drawing commands.
- L //draw to limits
- -0.5,-0.75 //remember, no spaces are allowed
GROUP ACTIVITY: Create a window-block of a 1 inch border with a width of 0.08 inches when plotted. Create a script file that takes this border imported at a scale of 1 in. = 100 ft., and plots it with a 1 inch border on a 11 in by 17 in. sheet. (Activity points: 0.5)
Transmitted GPS Signals
The principle of position determination by GPS and the accuracy of the positions strongly depend on the nature of the signals. A variety of criteria was considered in the development of a suitable signal structure. In consequence the GPS signal is quite complex and offers the possibility of determining the following parameters: one-way (passive) position determination, exact distance and direction determination (Doppler effect), transmission of navigation information, simultaneous receiving of several satellite signals, provision of corrections for ionospheric delay of signals and insusceptibility against interferences and multi path effects. In order to fulfil all these requirements, the signal structure described below was developed.
Choice of the carrier frequency
To transport data signals, a suitable carrier frequency is required. The choice of the carrier frequency is submitted to the following requirements:
- Frequencies should be chosen below 2 GHz, as frequencies above 2 GHz would require beam antennae for the signal reception
- Ionospheric delays are enormous for frequency rages below 100 MHz and above 10 GHz
- The speed of propagation of electromagnetic waves in media like air deviates from the speed of light (in vacuum) the more, the lower the frequency is. For low frequencies the runtime is falsified.
- he PRN-codes (explained below) require a high bandwidth for the code modulation on the carrier frequency. Therefore a range of high frequencies with the possibility of a high bandwidth has to be chosen.
- The chosen frequency should be in a range where the signal propagation is not influenced by weather phenomena like, rain, snow or clouds.
Based on these considerations, the choice of two frequencies proved to be advantageous.
Each GPS satellite transmits two carrier signals in the microwave range, designated as L1 and L2 (frequencies located in the L-Band between 1000 and 2000 MHz).
Civil GPS receivers use the L1 frequency with 1575.42 MHz (wavelength 19.05 cm). The L1 frequency carries the navigation data as well as the SPS code (standard positioning code). The L2 frequency (1227.60 MHz, wavelength 24.45 cm) only carries the P code and is only used by receivers which are designed for PPS (precision positioning code). Mostly this can be found in military receivers.
Modulation of the carrier signals
C/A and P-Code
The carrier phases are modulated by three different binary codes: first there is the C/A code (coarse acquisition). This code is a 1023 “chip” long code, being transmitted with a frequency of 1.023 MHz. A “chip” is the same as a “bit”, and is described by the numbers “one” or “zero”. The name “chip” is used instead of “bit” because no information is carried by the signal. By this code the carrier signals are modulated and the bandwidth of the man frequency band is spread from 2 MHz to 20 MHz (spread spectrum). Thus the interference liability is reduced.
The C/A code is a pseudo random code (PRN) which looks like a random code but is clearly defined for each satellite. It is repeated every 1023 bits or every millisecond. Therefore each second 1023000 chips are generated. Taking into account the speed of light the length of one chip can be calculated to be 300 m.
Pseudo Random Numbers (PRNs)
The satellites are identified by the receiver by means of PRN-numbers. Real GPS satellites are numbered from 1 – 32. To WAAS/EGNOS satellites and other pseudolites higher numbers are assigned. These PRN-numbers of the satellites appear on the satellite view screens of many GPS receivers. For simplification of the satellite network 32 different PRN-numbers are available, although only 24 satellites were necessary and planned in the beginning. For a couple of years, now more than 24 satellites are active, which optimizes the availability, reliability and accuracy of the network.
The mentioned PRN-codes are only pseudo random. If the codes were actually random, 21023 possibilities would exist. Of these many codes only few are suitable for the auto correlation or cross correlation which is necessary for the measurment of the signal propagation time. The 37 suitable codes are referred to as GOLD-codes (names after a mathematician). For these GOLD-codes the correlation among each other is particularly weak, making an unequivocal identification possible.
The C/A code is the base for all civil GPS receivers. The P code (p = precise) modulates the L1 as well as the L2 carrier frequency and is a very long 10.23 MHz pseudo random code. The code would be 266 days long, but only 7 days are used.
For protection against interfering signals transmitted by an possible enemy, the P-code can be transmitted encrypted. During this anti-spoofing (AS) mode the P-code is encrypted in a Y-code. The encrypted code needs a special AS-module for each receiving channel and is only accessible for authorized personnel in possession of a special key.
The P- and Y-code are the base for the precise (military) position determination. Since January 31, 1994 the AS-system is operating continiously and the P-code is only transmitted as Y-code.
Transmission of data
In the GPS system data are modulated onto the carrier signal by means of phase modulations. Phase modulation is a rarely used technique, compared to amplitude modulation (AM) or frequency modulation. In the following, these three modulation techniques shall be explained shortly.
For the amplitude modulation the amplitude, which corresponds to the strength of the signal, is changed in accordance to the data signal that shall be transported. If this principle would be applied to sound waves, the sound level would change in order to transport a signal. With increasing attenuation it becomes more and more difficult to filter the data from the signal. This kind of modulation is known from AM radio (that's what AM stands for: amplitude modulation).
For the frequency modulation, the carrier frequency itself is changed by modulating the data signal onto it. If we stay with the example of the sound waves, the pitch of the tones would be changed while the volume would be kept constant. Frequency modulated signals are less susceptible for disturbances and provides a higher bandwidth than AM modulation. This kind of modulation is used for FM radio.
When a data signal shall be modulated onto a carrier signal by phase modulation, the sine oscillation of the carrier signal is interrupted and restarted with a phase shift of e.g. 180°. This phase shift can be recognized by a suitable receiver and the data can be restored. Phase modulation leads to an extension of the frequency range of the carrier signal (leading to a spread spectrum) depending on how often the phase is shifted. When the phase changes, wave peaks are followed by wave minimums in a shorter distance than were in the original carrier signal (as can be seen in the graph).
This kind of modulation can only be used for the transmission of digital data.
The following graph shows the composition of signals which are transmitted by GPS-satellites. The setup of the NAV/System data is explained in the chapter "data signal composition".
Remark: Modulo 2 Sum means that sums are formed according to arithmetic rules. If the result is larger than 2, only the rest is kept which can not be divided by 2 (0+0=0; 0+1=1; 1+0=1; 1+1=0).
The GPS Tracklog
The GPS Tracklog in ArcPad is stored in a shapefile format. The GPS Tracklog can be started or activated when the GPS is active. ArcPad automatically records each GPS position it receives as a point feature in the GPS Tracklog shapefile, as long as the GPS Tracklog is running and the GPS is active. The GPS Tracklog is an electronic breadcrumb trail that shows the path that you have traveled. ArcPad uniquely displays these GPS positions, or points, in the tracklog as a red line.
The GPS Tracklog points are always captured in latitude and longitude degrees using the WGS84 datum. ArcPad automatically projects the tracklog points using the projection of the current ArcPad map when displaying the tracklog. Although ArcPad treats the GPS Tracklog point shapefile in a unique way, the tracklog is still a standard PointZM shapefile that can be used in the same way as other PointZM shapefiles. The Tracklog shapefile can be added to an ArcPad map by using the Add Layer(s) dialog box.
When adding the tracklog shapefile with the Add Layer tool, ArcPad treats the shapefile as a standard point shapefile. ArcPad displays the points as points and does not perform on-the-fly projection of the tracklog’s point data—which is in latitude and longitude degrees using the WGS84 datum.
For each point in the tracklog shapefile, ArcPad captures an x, y, and z coordinate with a user-specified m value. ArcPad also captures a variety of information received from the GPS receiver and stores this information in the attributes associated with the point. These attributes can be viewed by using the Add Layer tool to add the tracklog shapefile to an ArcPad map and then by using the Identify tool to display the selected point’s attributes.
The GPS information captured for each point in the tracklog, and the associated field, is as follows:
LATITUDE: Latitude in the datum of the GPS receiver
LONGITUDE: Longitude in the datum of the GPS receiver
ALTITUDE: Altitude in the datum of the GPS receiver (in meters)
EASTING: UTM easting
NORTHING: UTM northing
UTCDATE: UTC date
UTCTIME: UTC time
SOG: Speed over ground (in km/h)
COG_TRUE: True Course Over Ground (in decimal degrees)
COG_MAG: Magnetic Course Over Ground (in decimal degrees)
SATS_USED: Number of satellites used
HPE: Horizontal position error (in meters) (only when using a Garmin GPS receiver)
VPE: Vertical position error (in meters) (only when using a Garmin GPS receiver)
EPE: Estimated position error (in meters) (only when using a Garmin GPS receiver)
HDOP: Horizontal Dilution of Precision
VDOP: Vertical Dilution of Precision
PDOP: Position Dilution of Precision
QUALITY: GPS fix quality, where:
0 represents no fix.
1 represents GPS or SPS (Standard Positioning Service) fix.
2 represents DGPS (Differential GPS) fix.
3 represents PPS (Precise Positioning Service) fix.
DIFF_AGE: Age of DGPS fix (in seconds)
DIFF_ID: ID of DGPS station used
DEPTH : Depth (in meters)
DEPTH_OFF: Depth offset (in meters)
WATERTEMP: Water temperature (in degrees Celsius)
The size of the GPS Tracklog shapefile’s dBASE® table can get fairly large if the Tracklog is active for a long period of time. You can reduce the size by deleting fields from the tracklog shapefile’s dBASE (*.dbf) table. You can use ArcGIS Desktop (ArcView, ArcEditor, or ArcInfo) to delete fields in the shapefile’s dBASE table.
The x, y, and z coordinates, together with the m value, can be viewed by using the Geography tab of the point feature’s properties.
The GPS Tracklog points can be captured simultaneously while using the incoming GPS positions to capture other point, line, or polygon features. Consequently, the GPS Tracklog captures points independently from the ArcPad editing tools.
The GPS Tracklog layer is always displayed as the first layer on the Layers page of the Table of Contents.
The GPS Tracklog layer can be displayed, or made visible, whether the GPS is active or not. The GPS Tracklog has its own set of unique properties, which can be displayed by using the Layer Properties button in the Table of Contents.
Using these properties you can:
- Change the name and location of the Tracklog shapefile.
- View the number of GPS positions or points in the tracklog.
- Clear or delete all of the points in the tracklog.
- Specify the interval to be used by ArcPad when recording the GPS positions in the tracklog. An interval of 2, for example, means that ArcPad will only use every second incoming GPS position to record a tracklog point.
- Specify whether or not ArcPad must use the GPS Quality rules when capturing points in the Tracklog. The GPS Quality rules are specified on the Quality page of the GPS Preferences dialog box.
- Select which GPS information ArcPad must use to store in the tracklog point’s measure value. Available options are PDOP, HDOP, VDOP, TDOP, EPE, HPE, VPE, TIME, DEPTH, and SOG.
Any changes to the tracklog properties only take effect the next time the tracklog is started. Changes do not take effect if the tracklog is currently running.
The ArcPad map file (.apm) stores a list of the map layers and their properties in your ArcPad session. A map lists all of your layers together with the geographic extent and projection of the map. When you begin an ArcPad session, you can open an existing map or create a new map by adding layers of data or information.
You can display geographic information on a map as layers. Each layer represents a particular type of feature, such as streams, lakes, highways, political boundaries, or light posts. A layer does not store the actual geographic data; instead, it references the data contained in shapefiles, ArcPad AXF files, or images. The Table of Contents (TOC) button becomes active once a layer has been added to the current ArcPad map.
The Table of Contents displays all layers loaded in the current map, the GPS Tracklog, and the Map Grid layer. Within the Table of Contents, you can also change symbology using the legend, set snapping properties for editing, and select whether or not each layer can be identified or edited.
The screenshot below explains different tabs and icons on the Table of Contents window: